You have two ropes. Each rope can burn in exactly one hour. The ropes are not of the same length or width. Both ropes are also not of uniform width or thickness. The ropes are thick at certain places and thin at other places. Thus, a rope that is half burnt will not necessarily have taken 30 minutes to burn.
By burning the ropes, how do you measure exactly 15 minutes worth of time?
Let’s name the ropes as A and B.
Light A from both the ends and B from one end.
When A is finished burning we know that 30 minutes have elapsed and B has 30 minutes remaining. Now, light the other non-burning end of B and start the timer.
When B is finished burning, stop the timer as 15 minutes have elapsed.