Originally Posted by Coach A
7.. You have 2 ropes and a box of matches. You want to measure 60 minutes exactly using only the ropes and the box of matches. Here is what you know:
a) Each rope takes exactly 80 minutes to burn end to end.
b) The rope itself is made out of different material throughout the rope. So, it's rate of burning is not constant throughout the rope.
Can you measure exactly 60 minutes? If so, how?
We know you can measure 40 minutes by lighting both ends of the rope. So first you light one rope at two ends and the other rope on one end. Then when the first rope burn out, you'll know 40 minutes has passed and the second rope only has 40 more minutes to burn out. If you immediately light the second rope at the other end, that will measure half of 40 minutes which is 20 minutes. That way 40 minutes + 20 minutes = 60 minutes.