Measure 45 minutes with two burning ropes

Question

You have two ropes and a lighter. Each rope takes exactly 60 minutes to burn from one end to the other. However, the ropes are non-uniform: they may burn faster in some places and slower in others, so a rope that is half burned has not necessarily taken 30 minutes.

Using only these two ropes and the lighter, how do you measure exactly 45 minutes?

Accepted Answer

The key insight: lighting a rope from both ends makes it burn out in exactly 30 minutes, regardless of how non-uniform it is. The two flames together consume the whole rope's "60 minutes worth" of material, meeting somewhere in the middle after 60/2 = 30 minutes. Now the plan: 1. At time 0, light Rope A at both ends and simultaneously light Rope B at one end. 2. Rope A burns from both ends and finishes at exactly the 30-minute mark. At that instant, Rope B has been burning from one end for 30 minutes, so it has 30 minutes of burn time remaining. 3. At that moment (time 30), light the other en…

Measure 45 minutes with two burning ropes

Your answer

More Brainteasers questions