Question

Three men went into a second-hand shop to buy a

television.
It was priced in the window at £30.
Each of them handed over £10 to the shop assistant.
As the assistant opened the till, the manager had a quiet
word with him, "that TV is in the sale and is only £25
now, you will have to give them £5 back."
The assistant was very lazy and couldn't be bothered to
count out the right change for each man.
Instead, he took 5 £1 coins out of the till.
He put two of them in his own pocket and gave each
man £1 back.
Here's the problem:
The men have now paid £9 each for the TV.
The assistant has kept £2 for himself.
3 x £9 = £27.
£27 + £2 = £29.
But £30 was handed over in the first place.
WHERE IS THE MISSING £1?

Answer 1

Explanation:

The trick here is to realize that this is not a sum of the money that the three people paid originally, as that would need to include the money the clerk has ($25). This is instead a sum of a smaller amount the people could have paid ($9 × 3 people = $27), added with the additional money that the clerk would not have needed had they paid that smaller amount ($27 paid - $25 actual cost = $2). Another way to say this is, the $27 already includes the bellhop's tip. To add the $2 to the $27 would be to double-count it. So, the three guests' cost of the room, including the bellhop's tip, is $27. Each of the 3 guests has $1 in his pocket, totaling $3. When added to the $27 revised cost of the room (including tip to the bellhop), the total is $30.

To obtain a sum that totals to the original $30, every dollar must be accounted for, regardless of its location.

Thus, the sensible sum can be expressed in this manner: