Dolphins of the open ocean are classified as Type II Odontocetes (toothed whales). These animals use ultrasonic "clicks" with a frequency of about 55 kHz to navigate and find prey. You may want to review (Pages 465 - 467) . Part A Suppose a dolphin sends out a series of clicks that are reflected back from the bottom of the ocean 75 m below. How much time elapses before the dolphin hears the echoes of the clicks

0.1 sec

Explanation:

frequency of the clicks produced = 55 kHz = 55000 Hz

depth of the bottom of ocean from the dolphin = 75 m

we know that the speed of sound  in water is generally accepted to be ≅ 1480 m/s.

the total distance traveled by the sound from the dolphin, to the bottom of the ocean, and then back to the dolphin = 2 x 75 = 150 m

time elapsed will then be

time = distance traveled ÷ speed of sound

time = 150/1480 ≅0.1 sec