Calculate the minimum distance needed in water to hear the echo.

Given,

• Speed of sound in water (v) = 1500 m s^-1
• Minimum time to hear an echo (t) = 0.1 s

Let minimum distance to hear an echo be 'd'.

Then,

$$\begin{aligned}\text {Speed of sound} = \frac {2\times \text {Minimum distance to hear echo}}{\text { Minimum time to hear an echo}} = \frac{2\text d}{\text t} \\[1em] \Rightarrow 2\text d = \text v \times \text t = 1500 \times 0.1 = 150\ \text m \\[1em] \Rightarrow \text d = \frac{150}{2} = 75\ \text m\end{aligned}$$

So, minimum distance to hear an echo is 75 m.