The speed of a motor boat in still water is 20 km/h.It travels 150 km downstream and then returns to the starting point. If the round trip takes a total of 16 hours, what is the speed (in km/h) of the flow of river?

Let the speed of the flow of the river = s

The speed of motor boat in still water(m) = 20 km/h

Downstream speed = 20 + s

Upstream speed = 20 - s

Time taken for boat to travel 150 km downstream = $$\frac{150}{20+s}$$

Time taken for boat to travel 150 km upstream = $$\frac{150}{20-s}$$

Total time taken = 16 hours

$$\frac{150}{20+s}$$ + $$\frac{150}{20-s}$$ = 16

150[$$\frac{20-s+20+s}{(20+s)(20-s)}$$] = 16

$$\frac{150\times40}{16}$$ = (20+s)(20-s)

375 = 400 - s$$^2$$

s$$^2$$ = 25

s = 5 km/h

The speed of the flow of the river = s = 5 km/h

Hence, the correct answer is Option C