When computer programmers dabble in making change

My colleague who dabbled in economics when deciding how many lunch vouchers to buy had a number of other money-related quirks.

One of the ones that I remember is that when paying for a purchase, my colleague would double the balance and give the cashier that much money. For example, if the total was $5.20, my colleague would hand over $10.40.


Just to see if the cashier reacted when pressing the Enter code appeared to have no effect.

Total is $5.20.

Cash tendered is $10.40.

Change is $5.20.

Most of the time, the cashier wouldn't pay any attention. Heck, the cashier wouldn't even question why my colleague handed over such a strange amount of money.

Sometimes my colleague would mix it up and instead add $6.66 to the total. For example, if the total was $5.20, my colleague would hand over $11.86, just to see the cashier's reaction when the cash register indicated that the change due was $6.66.

And then one day, magic happened: The total was $6.66. Without skipping a beat, my colleague handed over $13.32.

Comments (51)
  1. Someone You Know says:

    Last Halloween, my wife unintentionally ordered $6.66 worth of fast food while wearing a devil costume.

    The cashier didn’t get it.

  2. cashiered cashier says:

    This strikes me as petty harassment of someone in no position to retaliate.

    It has been nearly 40 years since I worked a cash register, but I don’t remember it as creating an excess of joy that needed to be beaten down by customers’ games.

  3. Danny says:

    And? What happened? Your stories sometime Raymond are ending in a fish tail, give us the punch line. It is not the time of the 2200 when the TV’s shows are customer interactive and we decide what is the end, you’re the writer of the script, give us the conclusion(s).

    [The punch line was that he handed over $13.32. (When you hear the the joke whose punch line is “I’m a frayed knot,” do you ask, “But how did the bartender respond?”) -Raymond]
  4. Danny says, "Math is hard. Let’s go shopping!"

  5. nathan_works says:

    your colleague, raymond, sounds like a jerk. (I note you don’t call him a friend)

    I struggle to find an altruistic reason for such actions. It’s the kid with the magnifying glass burning ants. Purposefully doing annoying things to someone else, that’s called being a jerk.

    "We switched person A’s coffee with brand X, let’s see if he notices !"

  6. Random832 says:

    @cashiered cashier/nathan_works, why would this automatically be considered a bad/annoying thing to do?

  7. your colleague, raymond, sounds like a jerk

    As a one-time cashier I can promise that the cashier is almost certainly not offended.  Vaguely amused, perhaps, in a let-the-baby-have-his-bottle kind of way.

    The people behind him in line might feel differently, as he’s hunting around for that extra penny.

  8. W says:

    Doesn’t the cashier usually just hit the key for exact payment and calculate the difference himself? That’s the behavior I usually observe.

    But of course that might be different when they get handed such unfitting payments.

    [Teenagers in your part of the world must be smarter than ours. -Raymond]
  9. Matt says:

    A friend of mine worked in a shop in high school where customer’s totals would semi-frequently hit 6.66.  He was always amused by the folks who would choose to buy something else instead of paying the Number of the Beast.

  10. S says:

    My favorite is having an order that’s a few pennies over a dollar value, giving the cashier the next largest denomination, waiting for them to punch it in, and then pulling out the exact change thinking that I’m making it easier.

    It sad that on more than one occasion I’ve had the cashier call over the manager and ask "can he do this?".  I’m somewhat relived that the manager can at least do the calculation in their head quickly.

  11. porter says:

    > This strikes me as petty harassment of someone in no position to retaliate.

    So is saying "Have a nice day" to everyone.

  12. Mike Dunn says:

    I’ve done a similar experiment with $2 bills, which exist in the US but are rarely seen for whatever reason. I get a bunch of them from a bank, then use them at various stores just to see the cashiers’ reactions, since there is usually no place in the cash drawer to put them.

    The best reaction I’ve had was as a coffee shop, I paid for a $2.50 drink with a $2 bill and two quarters. As I was walking away, I heard the guy go "oh…" like he just realized he had a $2 and didn’t know what to do with it.

    $1 coins (which are also rarely seen in some parts of the country) can also be used for this experiment.

  13. Tim says:

    When I was a cashier lots of people would come by and buy something that was, say 3.75, hand me a 5 dollar bill, and then be like "oh, and here’s the quarter".

    So, I’d hand the quarter right back to them, and then another…

    Some people were pretty surprised.

  14. Mark (The other Mark) says:

    Two strings walk into a bar.

    "What’ll it be, fellas?" the Bartender asks.

    "I’ll have a beer" the first string replies.

    "Same for me.0gh8lujhil%" says the second string.

    "What was that?" asks the bartender.

    The first replies "Pardon my friend, he’s not null terminated"

  15. cashiered cashier says:

    @Raymond: On shelves at home I have a reprint of a book on education, published during WW II, which mentions in passing that many persons suddenly pulled into the retail trade had a hard time making change by the method you mention in the linked posting.

    @porter: when phatic communication is outlawed, only outlaws will say "Have a nice day." If a minimum-wage cashier following company policy strikes you as harassment, well, thank goodness for self-serve checkout and on-line shopping.

  16. nathan_works says:

    Random832 – because you’re purposefully playing games/"gotchya" with and patronizing/attempting to trick the person running the register. It’s rude.

  17. Philip says:

    Another money-related quirk: http://ln.hixie.ch/?start=1190803943&count=1 – using the low bits of a restaurant bill for data storage.

  18. Joel F says:

    @nathan_works: I used to be a cashier many years ago. I can see how you view this as rude, but there are far, far greater levels of rudeness.

    Even if I thought some customers were jerks, they still break up the monotony.

    Now, it could be mean if one found a cashier who takes the 6.66 thing very seriously and consistently tender an amount so that one’s change is 6.66. They’ll start doing math in their head pretty quick.

  19. Sy says:

    <quote>[Teenagers in your part of the world must be smarter than ours. -Raymond]</quote>

    No doubt!

  20. Kenneth says:

    What’s this fascination with carrying cash around? Seems cumbersome to me.

  21. Wavel says:

    I save my quarters in a big change jar so I like to pay for my purchases with enough money to get back quarters. If something is $1.28, i’ll pay $2.03 to get the 3 quarters. I’m always amazed at how confused the cashiers get at this. Actually, i’ve stopped being amazed.

  22. ERock says:

    @Philip: I try to tip so that my total charge has a digit-by-digit checksum of 9, although reading that link I think I might continue to do this and use one of the digits to rate the food from a scale of 0 – 9.

  23. porter says:

    > What’s this fascination with carrying cash around? Seems cumbersome to me.

    You may have noticed that as people carry less cash they loose connection with holding money, it’s no longer tangible, it becomes just a number. Then their use of credit and hence debt increases. Have you noticed any problems in the current economy where people have dug themselves into debt with little understanding of the consequences or the magnitude of the problem?

  24. Neil (SM) says:

    @porter: not true.  I rarely carry cash and don’t carry credit-card debt either.  Just more convenient.

  25. Bob Snyder says:

    I worked in a grocery store during my college years. One day a smiling old man came through my checkout line. As we finished up I playfully said "Thanks, and here is your receiPt." with emphasis on the P. Without missing a beat, he calmly replied "The word receipt is like a bathing suit. In both the P is silent.".

  26. configurator says:

    It’s been a really long time since I’ve seen a cashier use the computer to calculate change. I can’t remember a single one in the past year. The oddest thing is with my butcher – whenever I pay him by credit card, he adds a few Euros and gives me change for some reason. This is very convenient because he’s right before the store where I always need 1.43€, and that way I always have coins for them.

    • In restaurants and bars, the waiter goes to the cash register anyway so god knows what he’s doing.
  27. Gabe says:

    I don’t see why people are offended by forcing the cashier to make change in the same amount as the charge. Is it only acceptable to give exact change? Or only get change from round numbers of money? Or some third option I can’t imagine?

    I imagine that if I were a cashier, I would be more annoyed at somebody who always had to search for exact change than somebody who made change in novelty amounts.

  28. I save my quarters in a big change jar

    If you save dimes instead you’ll get more money per unit volume.

  29. I don’t see why people are offended by forcing the cashier to make change

    It’s the IRL equivalent of writing your for loops like this:

    for (int i = 0; i < 2 * max; i += 2) {

       foo(i / 2);


    It’s harder to type, it makes debugging one quantum more difficult, and it serves no benefit at all.  But I suppose it makes the person doing it feel more clever.

    So, fundamentally, it’s rude, using the "putting your own whims before the common good" definition of rudeness.

  30. Dave says:

    That’s a great example, Maurits.  

    I had the misfortune of spending a summer working part-time as a cashier as a teenager and still remember the annoying customers who insisted on trying to be clever by giving strange payment amounts.  The blank stares that cashiers give back to these customers aren’t due to confusion but rather because the cashier would be fired if he or she expressed what they’re really thinking.

    For the record, if you give a cashier something like $2.03 to pay for $1.28, they don’t look at you confused because they can’t figure out that the change is three quarters.  They look at you confused because they think you’re too stupid to figure out how much you were supposed to pay.  Typically when a customer digs in their pockets for pennies, it’s to produce exact change, not because they are obsessive–compulsive about their pocket change.  

  31. Your colleague is a bad, bad person.

  32. jeremy says:

    I honestly can’t comprehend why you people are so offended at this. Do you suck at arithmetic so much that this concept just blows your mind?

    Do you get angry if someone uses "effect" when everyone else thinks they should use "affect," but really "effect" was the right word to use?

  33. Gabe says:

    Sorry, but can somebody please explain why novelty change amounts like $6.66 or $12.34 are offensive, while $7.82 wouldn’t be (unless the initial charge was $7.82).

    Perhaps somebody could write an algorithm for how I can tell whether a change amount will be offensive.

  34. Worker says:

    I always, if possible, made sure to type the accurate tendered amount into the machine.  This made sure that disputes were easy to resolve.  Also, it improves speed and accuracy.

    And who cares how much people tender.  It makes no difference.  Unless your being evaluated on transaction speed.  I probably wouldn’t even have noticed such things — it really is a who cares.

  35. Sven says:

    I’m fairly certain that in the Netherlands, if you were to try this, the cashier would just say "why did you give me such a strange amount of money?" before even entering the amount on the cash register.

    If the total’s $5.20, and you give $10.40, there’s no reason for that. They’d give you back the pointless 20 cents before doing anything else.

  36. porter says:

    I’ve paid for things using postage stamps.

  37. Drak says:


    Indeed. If you were to pull any of this stuff in the Netherlands the cashier would look funnily at you and shove any excess money right back at you. Sometimes they don’t even want that 20 cents for the 5.20 if you pay with 10.

    Not that I ever use cash, I use my debit card (pinpas), which means I won’t owe anybody money because the lower limit on it has been set to 0.00 by me at some point.

  38. Worf says:

    Honestly, I do stuff like that when paying to get even change back. At the very least, when I’m paying, I strive to get at least a nickel back in place of pennies. If not (because I end up spending all my change), oh well, I take a pile of coins back.

    I never understood the whole penny jar concept – I use my pennies to convert into larger denomination coins.

    Like $1.28, I’d pay $2.03 to get rid of 4 coins ($2, 3 pennies) and get 3 quarters back.

    Anyhow, I think the current rudest thing a cashier gets these days are those who are so glued to their phones, they’re yapping through the lineup and the checkout. Not even a hello or thank you. They also are slowest to pay as they juggle phone and wallet/purse.

  39. Dave says:

    You still use single cents?  How qaint.

  40. cashiered cashier says:

    Heck, I give $2.03 on a $1.28 cost all the time. And the places I usually go have registers that do the arithmetic.

  41. keith says:


    It’s not strongly rude or offensive, but the whole thing seems like a borderline Asperger Syndrome behavior, compulsively indulging in creating pretty mathematical patterns for personal mental enjoyment without empathy (or in the original post, anti-empathy as it was an attempt to confuse or trick the cashier) for the external costs imposed on others to execute the pattern.  

    It’s more of a heuristic than an algorithm: if the amount tendered does not pattern-match with a widespread social norm (I would assert there are three: "whole bills", "exact change", and "no pennies in return change"), it’s obnoxious.

  42. Gabe says:

    Keith: I guess what I don’t understand is the external cost imposed on others. Given that the cashier is going to let the computer do the math and that I don’t make anybody wait while I find the change, there doesn’t seem to be any external cost.

  43. Boris says:

    The day I realized that NJ Transit buses accept pennies, I finally got rid of most of the ones I had accumulated. Honestly, I think I’ve paid an entire bus fare in pennies a couple of times. I still use up the odd pennies I happen to collect on the bus.

  44. keith says:


    If your given assumptions are true, then yes, there is no cost (though, I have a real life counter example below).  If the given assumptions are not true, the breaking of a social norm is like the breaking of a stored optimized query plan.  

    NCR implemented a single button for the "$20 tendered" use case on the touch-screen cash register at the convenience store I went to this morning: is this a premature optimization of the business process or an effective user-driven feature?  

    I assert that there is a social norm that cashiers generally accept when a customer tenders an amount that minimizes wallet entropy.  The benifit to the payer is obvious and is worth the additional cost to the cashier by social norm.  If I tender an amount that yields a non-obvious benefit to me and is also not even bills, and the cash register is implemented with a feature like above, then yes, I have indeed put a minor cost on the cashier: 5 data entry keystrokes instead of 1, each introducing a small possibility of data entry error, compounded with cognitive dissonance from the breaking of the norm.

    To tie this to software development, I have found the HCI work I’ve done to be among the most important for system success.  

  45. Perhaps somebody could write an algorithm for how I can tell whether a change amount will be offensive.

    Here you go.  Some defined terms, first:

    Total: what you owe.

    Tender: what you give the cashier.

    Change = Tender – Total: what the cashier gives you.

    Every dollar amount can be realized with a wide variety of combinations of bills and coins.  One of these combinations is "canonical" meaning that it uses the least total number of bills and coins.  For example, $0.41 can be tendered with four dimes + one penny, or with one quarter + one dime + one nickel + one penny.  The latter is canonical.

    A "tender" COMBINATION is "offensive" if canonical change contains any of the same bills or coins that the tender did.

    NOTE: offensivity is a property of the combination, not necessarily of the amount.

    Some examples:

    Total: $5.20

    Tender: a $5 bill

    Change: three quarters + a nickel

    Not offensive.

    Total: $5.20

    Tender: four $1 bills + four quarters

    Change: three quarters + a nickel

    Offensive.  The cashier hands three of your quarters back to you.

    Total: $6.66

    Tender: one $10 bill + three $1 bills + three dimes + two pennies

    Change: one $5 bill + one $1 bill + two quarters + one dime + one nickel + one penny

    Offensive.  The cashier hands back one of your $1 bills, one of your dimes, and one of your pennies.

    Total: $6.66

    Tender: one $10 bill + two $1 bills + two quarters + two dimes + one penny ($11.71)

    Change: one $5 bill + a nickel

    Not offensive.

  46. Joe says:

    Then their use of credit and hence debt increases.

    Give my wife cash and it vanishes into the ether; make her use a debit and credit card and she becomes almost (ok–closer to) my own, uh, scrooginess.

  47. James says:

    @jeremy: It’s rude because if purchasers are capable of paying exactly twice the amount, they very likely could also have just paid exact change.  Meanwhile they’re making the cashier spend the time to actually make the change.  This wastes the cashier’s time (and the time of anyone else in line).

    It’s takes only a few seconds seconds, but the fact that someone’s doing it intentionally is what makes it galling.

    @Neil (SM):

    Just because it’s not true for you doesn’t make it not true for other people.  A lot of people do need the affordance of physical cash.  Also, debit cards often have overdraft fees that people need to worry about.

    @Mike Dunn v2.0:

    You’ve seen http://www.snopes.com/business/money/tacobell.asp , right?

  48. Too early in the morning for math.  Belay those examples.  Replace them with:

    Total: $5.20

    Tender: a $5 bill + a $1 bill

    Change: three quarters + a nickel

    Not offensive.

    Total: $5.20

    Tender: a $5 bill + ***four quarters***

    Change: ***three quarters*** + a nickel

    ***Offensive*** the cashier has to hand three of your quarters back to you.  You’ve wasted his time and yours.

    Total: $6.66

    Tender: one $10 bill + ***three $1 bills*** + ***three dimes*** + ***two pennies***

    Change: one $5 bill + ***one $1 bill*** + two quarters + ***one dime*** + one nickel + ***one penny***

    ***Offensive*** the cashier hands back one of your $1 bills, one of your dimes, and one of your pennies.

    Total: $6.66

    Tender: one $10 bill + two $1 bills + two dimes + one penny ($12.21)

    Change: one $5 bill + two quarters + a nickel

    Not offensive.

  49. Kimmo says:

    As already mentioned (@Keith), this is a type example of Asperger. I was humored by the mathematical pattern and the “leet” punch line. Then I read in the comments that this was rude behavior. I didn’t know. It never had those bearing, just mathematical patterns that soothes…

    Being a bona fide Asperger , I have adjusted as best I can and do for most part my mind experiments in my head rather than acting out, then again, that being my honest effort I’m quite uncertain of the success rate. For trying my best, yet constantly failing I would expect for people in my environment to please correct me. FYI: Asperger people varies quite as much as people in (take your pick) assembly.

  50. Azarien says:

    Maurits: not so offensive, since if you gave $5 and three quarters for $5.20 total, the cashier would just ignore two of the quarters and take only $5.25

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