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up vote 13 down vote favorite
1

When I was in college, each semester there were companies that would buy back your used textbooks. One semester I brought my books to the book-buyer, who said "We'll pay you $41". He grabbed a thick stack of one-dollar bills and, with the thumb of his other hand, "buzzed" off a stack of them in less than 1/2 a second. He handed them to me. Sure enough, there were 41 one-dollar bills in the stack!

one dollar bills

It took me a year to figure out how he did it so quickly. How did he do it?

HINT:

The bills were new.

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Long long ago, the same effect was puzzling me for long months as a child (preschool, I suppose). I used to observe that trick performed by newsstands sellers, when my parents were buying tickets for city trams and buses. The sellers were able to 'immediately' take a required amount of tickets from a pile and I didn't even realize there were serial numbers on them. :) – CiaPan 5 hours ago
up vote 22 down vote accepted

He had gotten them

fresh from the mint/bank in printed order

so that he could easily count the

Serial numbers on them

going up to 41

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5  
Half a second though? – greenturtle3141 8 hours ago
    
@greenturtle3141 "Half a second" was an exaggeration. I wanted to emphasize that the book-buyer didn't do any actual counting. – CreatedByBrett 6 hours ago
2  
Oh. I considered this possibility but rejected it on the grounds that half a second was probably too short for it to work. – Gareth McCaughan 3 hours ago
    
Counted in a jiffy, this time literally :) – ABcDexter 49 mins ago
up vote 13 down vote

This was actually done by

Simply counting and pre-bundling dollar bills in stacks of seemingly random amounts such as 41. The buyer would then name one of those numbers as his quote for any book that reached his counter, grab the corresponding stack from his drawer, and buzz through it as described above to create the illusion that he is an extremely proficient money-counting machine.

Why the effort, you ask? Well, the buyer obviously

hoped that his little stunt would one day be noticed, admired, and eventually recorded in the annals of history via sites such as this one.

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up vote 11 down vote

Suppose the bills are

in "sub-stacks" of, say, 10, each stack slightly sheared so that a small thumb movement releases 10 bills at a time. (For an exaggerated picture of the geometry I have in mind, imagine a vertical stack of Zs.)

Then the book-buyer could

"count" off four blocks of 10 almost instantly, and then change the angle of "buzzing" to get one more bill. He'd have been markedly slower if he'd been paying you \$49 instead of \$41.

(I don't find this answer terribly convincing and suspect there's something subtler afoot.)

share|improve this answer
    
that z thing is the only trick I could think of. But considering the hint, I assumed it was something else. – stack reader 10 hours ago
3  
If only you were a stack counter instead of a stack reader, you would surely know the answer! – Gareth McCaughan 10 hours ago
    
Very clever! But I don't think that's The Answer. – CreatedByBrett 6 hours ago

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