Question

A cargo ship has the job of transporting goods from city A to city B on the downstream and come back upstream. If the velocity of ship in still water is 30 km/hr and velocity of the river stream is 15 km/hr. Find the average speed of the ship in the journey.

• A. 30 km/hr
• B. 20 km/hr
• C. 45 km/hr
• D. 22.5 km/hr

Answer 1

The correct option is D

22.5 km/hr

While going downstream the velocity of ship will be 30 + 15 = 45 km/hr
While going upstream the velocity of ship will be 15 km/hr
Now distance is same between cities. Let's assume that to be d

For downstream, v=45
d=d
$t=\frac{d}{45}$

Upstream, v = 45
d = d
$t=\frac{d}{15}$
Avg. speed=$\frac{d+d}{\frac{d}{45}+\frac{d}{15}}$