Description for Correct answer:
Time taken in covering 7.5 km upstream = \( \Large \frac{3}{2} \ hours \)

\( \Large \therefore \ Time \ taken \ in \ covering \ 7.5 km \ downstream = \frac{3}{4} \ hours \)

\( \Large Speed = \frac{Distance}{Time} \)

\( \Large \therefore \) Rate downstream of boat

= \( \Large \frac{7.5}{\frac{3}{2}} = (\frac{7.5 \times 2}{3}) kmph \)

= 5 kmph

Rate downstream of boat

= \( \Large \frac{7.5 \times 4}{3} \) = 10 kmph