A submarine in the sea, sends ultrasonic ping and a stopwatch is started simultaneously. The stopwatch stops on receiving the reflected wave from an obstacle…

Given,

Time taken = 1 minute 40 seconds = 100 s

Speed of sound = 1500 ms^-1)

So,

$$\begin{aligned}\text {Distance of the obstacle} = \frac {\text {Speed of sound} \times \text {Time taken}}{2} \\[1em] =\frac{1500 \times 100}{2} = \frac{150000}{2} \\[1em] = 75000\ \text m = 75\ \text {km}\end{aligned}$$