Suppose that you clapped 15 times in 10 seconds and that each clap was in tempo with the preceding echo. Thus a single clap would go to the surface and back in 2/3 second (2/3 second X 15 = 10 second). Sound travels 220 meters in 2/3 second. Since the sound made a round trip to the surface and back, the distance to the surface is one-half of 220 meters, or 110 meters.