A man strikes one end of a thin rod with a hammer. The speed of sound in the rod is 15 times the speed of sound in air.
A woman, at the other end with her ear close to the rod, hears the
sound of the blow twice with a 0.12 s interval between; one sound comes through the rod and the other comes through the air along side the rod. If the speed of sound in air is $343 \mathrm{m} / \mathrm{s},$ what is the
length of the rod?