Less than a minute
There are two ropes both are of different length. These ropes can be burnt in equal amount of time that is 1 hour each. You cannot fold the ropes. The rate of burning of rope is constant. Now the puzzle is you have to burn both the ropes exactly in 45 mins. What will you do?
Light one of the rope from both sides and the other from one side only. Now that we know rope which burns from both side will burn in half an hour. So lets wait for half an hour. Here we don’t have clock half an hour is measured only by burning of first rope.
After exactly half an hour the first ropes will be burned totally, at the exact time the second rope will be burned half which means it can burn for another half hour. Now light the other side of the second rope. Its burning time will be half that is half of half hour in 15 min.
Thus total time taken to burn both the ropes is 45 Min.