Answer
0.12 s
Work Step by Step
If pulses were to be sent out too frequently, a pulse would return before the next one was sent. The minimum time between pulses is the time it takes a pulse to make a round trip, down and back, to a depth of 85 m.
The speed of sound in fresh water is 1440 m/s.
$$t=\frac{d}{v}=\frac{170m}{1440m/s}=0.12s$$