Puzzle 98: Timing with Ropes
You have two ropes, each of which takes two hours to burn if lit at one end. These ropes are not homogeneous and some parts of the rope burn more quickly than the other parts. If you cut the rope into half, you cannot assume that the half rope will take one hour to burn.
Use these ropes to measure 1hr 30 min.
Extension:
You have a piece of string which when set on fire from one end takes 120 minutes to burn completely. It has a property that the time taken to burn a distance D from one end is the same as the time taken to burn a distance D from the other end. The rate of burning is not uniform – different parts of the string will take different amounts of time to burn.
You need to boil an egg for exactly 15 minutes using the string as a timer. How will you do it?
Use these ropes to measure 1hr 30 min.
Extension:
You have a piece of string which when set on fire from one end takes 120 minutes to burn completely. It has a property that the time taken to burn a distance D from one end is the same as the time taken to burn a distance D from the other end. The rate of burning is not uniform – different parts of the string will take different amounts of time to burn.
You need to boil an egg for exactly 15 minutes using the string as a timer. How will you do it?
Source: http://www.testfunda.com