Question

A motorboat travels 30 km downstream in 3 hours and 40 km upstream in 5 hours. Find the speed of the boat (in km/hr).

• A. 9 km/hr
• B. 1 km/hr
• C. 2 km/hr
• D. 4.5 km/hr

Answer 1

The correct option is A

9 km/hr

Let the speed of boat be S1 and the speed of the water stream is S2.

If the stream and boat are moving in the same direction (downstream or along the stream), the effective speed of the boat = S1 + S2

And, if the stream and boat are moving in the opposite direction (upstream or against the stream), the effective speed of the boat = S1 - S2

The speed of the boat along the stream (downstream) = S1 + S2 = $\frac{30}{3}$ = 10 km/hr ---------(1)

The speed of the boat against the stream (upstream) = S1 - S2 = $\frac{40}{5}$ = 8 km/hr ---------------(2)

We have two equations now, and we need the speed of the boat i.e., S1

So, (1) + (2) will give us 2(S1) = 18 km/hr or S1 = 9 km/hr