There seem to be a lot of comments hung up on the length of the ropes. The whole point is to discard the physical dimensions and consider only the TIME. Even if a rope were constructed so that the first 80% of it’s length burnt in 30 min, and the last 20% in the remaining 30 minutes, by lighting both ends at once the entire rope will still be consumed in exactly 30 minutes (although the flames will meet far from the center). With this principle in mind, the second step for the 15 minute measurement should follow without much difficulty.