Well this puzzle is asked in most of the interview questions. You are given two ropes of same width and length.The two ropes will burn completely in 1 hour.
You have to estimate 1.5 hours using the ropes.Please note that the ropes are not of uniform cross section. So it may happen that 60% of rope burns in 0.5 hour and the remaining in 0.5 hour.

Well the solution is simple.First take one of the rope and put it’s both the ends on fire.Since it takes 1 hour to completely burn a rope but if you put the fire from both the ends it will take only 0.5 hour to burn it.
As soon as the 1st rope is burnt completely take the second rope and put it on fire. it will take 1 hour to burn completely.
So in total we can measure 1.5 hours using both the ropes.