Description for Correct answer:
Speed of boat in still water = x kmph.

Speed of current = y kmph.

Rate downstream = (x + y) kmph

Rate upstream = (x - y) kmph,

According to the question,

\( \Large x + y = \frac{20}{1 \frac{40}{60}} \) kmph

=> \( \Large x + y = \frac{20}{\frac{5}{3}} = \frac{20 \times 3}{5} \)

= 12 kmph ....(i)

\( \Large \therefore \frac{2 \times 20}{x + y} = \frac{20}{x - y} \)

=> 2x - 2y = x + y

=> x = 3y ....(ii)

From equation (i),

3y + y = 12

=> \( \Large 4y = 12 => y = \frac{12}{4} = 3 \ kmph \)

\( \Large \therefore x = 3 \times 3 = 9 \ kmph \)

\( \Large \therefore \) Rate upstream

= (x - y) kmph

= 9 - 3 = 6 kmph