Three guys go to the bar and get £30 worth of drinks. They pay £10 (10*3=30) each and the waitress takes the money. Before she puts it in the till the manager notices the guys and tells her “I know these guys, give them a £5 discount”
On the way to their table the waitress decides to give the guys £3 back and keep £2 as a tip.
The guys take a pound each, so instead of paying £10 each they end up paying £9 each (9*3=27).
And the question is: if they ended up paying £27 and the waitress kept £2 where did the last pound go?
In: 397
“where did the last pound go” is a deception.
It is an invalid question. There *is no “last pound”.*
The story about the waitress bringing money back&forth is a distraction for the reader to let that deception go unnoticed.
The deception starts because the 2 pounds are added twice:
* paying £27 (25+**2**)
* the waitress kept £**2**
The “2” crops up twice, and that is the trick. The 2 pounds should only be accounted for once.
The question tricks you by secretly accounting for it twice to make it an imaginary 4 pounds , distracting you with a 5 pound refund payment (which is nominally similar to 4 pounds), and then asking you why the imaginary 4 pounds is different to the 5 pounds. However, this is not a valid comparison.
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You can look at it 2 ways:
Either:
1. Each man paid £10
2. They each got £1 back
3. So they paid £27 overall (NOT £30!)
4. The restaurant got £25 and the waitress got £2 (this correctly adds up to £27)
Or
1. Each man paid £8.333…
2. The waitress opportunistically stole £0.666… from each of them, by pretending the price was £9 each.
3. Combining the money they paid, and the money stolen from them, each man is down £9
4. The waitress stole £2 in total (3 * 0.666… = 2).
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