You have two lengths of rope. If you set fire to the end of either of them, the rope will burn in exactly one hour. They are not the same length or width as each other. They also are not of uniform width (they might, for example, be wider in the middle than at the end), thus burning half of the rope is not necessarily going to take 30 minutes. By burning the ropes, how do you measure exactly 45 minutes worth of time?