It is given that the speed of sound in a specific area inside the water is 1300 m/s. What should be the minimum distance between the source and reflector in water so that echo is heard distinctly?

The minimum time to hear the echo distinctly is 0.1 s.

$$\begin{gathered} \mathrm{Speed} \mathrm{of} \mathrm{the} \mathrm{sound}=\frac{2\times \mathrm{Distance} \mathrm{between} \mathrm{source} \mathrm{and} \mathrm{reflector}}{\mathrm{Time}} \\ \Rightarrow \mathrm{Distance} \mathrm{between} \mathrm{source} \mathrm{and} \mathrm{reflector}=\frac{\mathrm{Speed} \mathrm{of} \mathrm{the} \mathrm{sound}\times \mathrm{Time}}{2} \\ \Rightarrow \mathrm{Distance} \mathrm{between} \mathrm{source} \mathrm{and} \mathrm{reflector}=\frac{1300\times 0.1}{2} \\ \Rightarrow \mathrm{Distance} \mathrm{between} \mathrm{source} \mathrm{and} \mathrm{reflector}=65 \mathrm{m} \end{gathered}$$