End-of-week Maths brainteaser for your class…

Here’s a wonderfully simple, but surprisingly knotty brainteaser to try on your class.  Anyone who’s ever struggled with dividing up a restaurant bill will recognise this situation.  The maths should be basic arithmetic, and yet…

Three friends go to a restaurant. The bill is £30, so each guest pays £10. The waiter takes the money to the till and realises the bill should only be £25. So he puts £25 in the till and takes five pound coins back to the table. On the way back, the waiter realises that he cannot divide the money equally. As the customers didn’t know the total of the revised bill, the waiter decides to just give each guest £1 and keep £2 for himself.

Now that each of the customers has been given £1 back, each has paid £9, bringing the total paid to £27. The waiter has £2. If the customers originally handed over £30, what happened to the remaining £1?

Click here to download the question and solution in a Word file.

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