Puzzle 8

A string is wound symmetrically around a circular rod. The string goes exactly four times around the rod. The circumference of the rod is 4cm and its length is 12cm. Find the length of the string.

This question was originally set about 30 years ago in an exam given to 18-year-olds in 16 countries studying ‘advanced’ maths. (A-level and similar).

It was one of the most difficult questions in the test, with only 10 per cent of students overall getting the right answer.

Solution