Description for Correct answer:
Rate downstream

= 4x kmph

Rate upstream = 3x kmph

According to the question.

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

\( \Large \frac{70}{4x} = 3 \frac{1}{2} \)

=> \( \Large \frac{70}{4x} = \frac{7}{2} \)

=> \( \Large \frac{10}{2x} = 1 \)

=> 2x = 10 => x = 5

\( \Large \therefore Rate \ upstream = 5 \times 3 = 15 kmph \)

\( \Large \ Rate \ downstream = 5 \times 4 = 20 kmph \)

\( \Large \therefore \ Speed \ of \ boat \ in \ still \ water \)

= \( \Large \frac{1}{2} (20 + 15 ) \ kmph \)

= \( \Large \frac{35}{2} = 17.5 \ kmph \)s