If the speed of the stream is 20% of the speed of the boat in still water and it covers 120 km upstream in 150 minutes, then what is the downstream speed of the boat?

Let's assume the speed of boat in still water is 'B' and the speed of stream is 'C'.

If the speed of stream is 20% of the speed of the boat in still water.

C = 20% of B

$$C=\frac{20}{100}\times\ B$$

$$C=\frac{1}{5}\times\ B$$

$$\frac{B}{C}\ =\ \frac{5}{1}$$

So B = 5y and C= y.

Boat covers 120 km upstream in 150 minutes.

$$\frac{120}{5y-y}=\frac{150}{60}$$

$$\frac{4}{4y}=\frac{1}{12}$$

y = 12

Downstream speed of the boat = B+C = 5y+y

= 6y

= $$6\times\ 12$$

= 72 km/h