6.01: Two ropes. 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.
Light rope 1 on both ends, when the flames touch, light rope 2. When rope2 = 1/2 lenght, light other end of rope 2.