Brain Teasers/Measure 15 minutes using ropes that burns for one hour
You have two ropes, and each takes exactly one hour to burn. How would you use them to time exactly 15 minutes? Note that the ropes are of uneven densities, so half the rope length-wise does not necessarily take half an hour to burn.
Each rope takes one hour to burn. We can measure one hour by burning one rope.
If we start burning a rope at both end at once, the rope will burn for half an hour.
If we start burning a rope from one end, and start burning the other rope at both end. The second rope will burn for half an hour. Then we start burning the first rope from the other end. It will burn out in next 15 minutes.