Answer
The elapsed time between an emitted pulse and the return of its echo is $95.6~ms$
Work Step by Step
We can convert the depth to units of meters:
$d = 40.0~fathoms\times \frac{1.83~m}{1~fathom} = 73.2~m$
We can use $v = 1533~m/s$ as the speed of sound in seawater. The pulse must travel to the bottom and return to the surface for a total distance of $2d$. We can find the required time:
$t = \frac{2d}{v} = \frac{(2)(73.2~m)}{1533~m/s} = 0.0956~s = 95.6~ms$
The elapsed time between an emitted pulse and the return of its echo is $95.6~ms$.
