Uneven body thickness. We assume that the linear velocity of the burning rope is the same, that is, it has nothing to do with the thickness. Then cut off a part of the thicker part of the rope, such as "Tian" divided into two "Tian" (do you understand what I mean), so as to cut off 1/4 of the total length of the rope, that is to say, cut off the rope that needs to be burned for 15 minutes, and burn it in turn (one is burned first, and the other is burned less), so as to burn it out.