Description for Correct answer:
Rate downstream of boat = 4x kmph

Rate upstream of boat = 3x kmph

According to the question,

=> \( \Large \frac{70}{4x} = (3 + \frac{30}{60}) \ hours \)

=> \( \Large \frac{70}{4x} = 3 \frac{1}{2} = \frac{7}{2} \)

=> \( \Large 7 \times 4x = 70 \times 2 \)

=> \( \Large x = \frac {70 \times 2}{7 \times 4} = 5 \)

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

\( \Large \frac{1}{2} ( \ Rate \ downstream + \ Rate \ upstream ) \)

= \( \Large \frac{1}{2} \times 7x = \frac{7 \times 5}{2} \)

= 17.5 kmph