# Erk Erk Erk

**Calculus**Level 5

Suppose that two whales, some distance apart, play at the surface of the sea. One whale emits a cry, which travels toward the other whale along an arc that goes down 7.5 kilometers (without hitting the oceanâ€™s bottom). How long (in seconds) does the cry take to reach the other whale?

Assume that the speed of sound at depth D km is \( \frac{ D + 72.5 } { 50 } \) kilometers per second (so the speed changes even at the surface, contrary to the initial statement).

This problem is shared by Gary Antonick.

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