The Rope Trick

Rumour has it that this puzzle is set for prospective Ph.D students in some Californian university - Stanford maybe?

The question is this: You have two pieces of rope and a box of matches. You have to use the ropes to mark a period of 45 minutes. The ropes are not uniformly thick and they do not necessarily burn at a constant rate, and you have no way of knowing how fast any particular portion of rope will burn. All you know is that each rope will each take exactly an hour to burn from one end to the other. What do you do? Note that this problem does not require any kind of "thinking outside the box".

Show answer

Set light to one of the ropes at both ends and to the other rope at one end. When the first rope has burned up completely (which takes 30 minutes), light the other end of the second rope, which will then take a further 15 minutes to burn up