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
There is no last pound. They paid 9 each, so a total of 27. 25 pound went to the till and 2 pound was taken by the waitress, which adds up to 27 pound. Or in another way, they paid 27 pounds and got a 3 pound discount totalling the 30 pound of their original bill.
There’s a sleight of hand in the question.
The drinkers actually paid £27 and the waitress kept £2 of that as a tip.
Initial payment: £30
Actual money given back: £3
Actual payment: £30 – 3 = £27
‘Tip’: £2
Bar gets: £27 – 2 = £25
The guys paid £27
However, they weren’t paying £30. They were only paying £25 because of the £5 discount.
So they paid £25 to the bar, and £2 to the waitress (=£27)
Other explanations do a better job than I could, but it’s worth noting that there are a lot of cons that work by a similar principal – the over-simplified version is that if something is worth $5 and you pay for it with a $20, then “find” a 5 to get your $20 back, but the $5 you “find” is in part of the change… there are more steps added in to obfuscate this, but the gist is that so much money goes back and forth and you add and subtract enough that you “magically” end up with more money than you started with.
People are tricked into doing the wrong calculation. They subtract rather than adding.
They didn’t pay £30 and remove a £2 tip. They paid £25 and added a £2 tip.
The waitress kept £2 **from the £27**. £27 – £2 = £25, which is exactly what went to the bar. There’s no reason to add the £2 to the £27.
You start with $30 of beer, $30 for payment and $0 of change.
Then the manager changed it to $25 of beer. There’s still $30 in payment, with n $5 in change.
The waitress takes her tip. Now you have $25 of beer, $30 payment, $2 tip, and $3 change.
The whole riddle is basically just adding things wrong to confuse the reader.
A big part of math is learning how to translate problems from a spoken language into math and back. Algebra is hard and people are bad at it. That problem relies on people making a translation mistake, the guys paid 27 and got 3 back so the right calculation is either Total spent – change = Bill including tip (30-3=27) or pretip bill plus tip equals total spent (25+2 = 27) but the problem sets up the problem as 30-2 = 27 which is obviously wrong once translated to math, but it’s hard to see where you made the translation error.
This riddle changes the goal post and then confuses the answer with the original goal.
£30 was the original goal post, but the new goal became £25 with the discount. If we try this scenario, the guys each paid £9 totaling £27, if the original bill after discount was £25, where did the other £2 go? It becomes much easier to see whats going on when you aren’t changing the facts around.
I don’t see any problem.
guys initially give money out, and I use negative number for this: -30
restaurant received +25
waiter received +2
guys received +3
Computation/proof: -30 + 25 + 2 + 3 = 0. Everything is accounted for.
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