A boat covers a distance 24 km upstream and returns to the same point in a total of 4 hours. If the speed of boat in downstream is twice its speed in upstream, then the speed of boat in upstream is:

9 km/h

The correct answer is Option (3) → 9 km/h

Let speed of boat in upstream = $x$ km/h.

Then speed of boat in downstream = $2x$ km/h.

Time taken upstream = $\frac{24}{x}$

Time taken downstream = $\frac{24}{2x} = \frac{12}{x}$

Total time = $\frac{24}{x} + \frac{12}{x} = \frac{36}{x}$

Given total time = 4 hours

$\frac{36}{x} = 4$

$x = \frac{36}{4} = 9$

Therefore, speed of boat in upstream = $9 \, \text{km/h}$