Description for Correct answer:
Speed of current = x kmph (let)

Rate Upstream of boat B = (10 - x) kmph

Rate downstream of boat A

= 10 - x + x = 10 kmph

\( \Large \therefore \frac{40}{10 - x} - \frac{20}{10} = 3 \)

=> \( \Large \frac{40}{10 - x} = 5 \)

=> 50 - 5x = 40

=> 5x = 50 - 40 = 10

=> \( \Large x = \frac{10}{5} = 2 \ kmph \)

\( \Large \therefore \)Speed of boat A in still water = 10 - 2 = 8 kmph