Question

A boat covers 24 km downstream in 2 hours and 12 km upstream in 2 hours. Find the speed of the boat in still water.

• A. 9 km/h
• B. 8 km/h
• C. 7.5 km/h
• D. 10 km/h

Hint:

Set up two simple equations for boat speed and stream speed from the downstream and upstream speeds, then eliminate one variable by adding them.

Answer 1

The Correct Option is A

Step 1: Let the boat's speed in still water be $b$ km/h and the stream's speed be $s$ km/h.
Step 2: Downstream: $b+s=\dfrac{24}{2}=12$ ...(i). Upstream: $b-s=\dfrac{12}{2}=6$ ...(ii).
Step 3: Adding (i) and (ii): $2b=18\Rightarrow b=9$ km/h. (Subtracting gives $s=3$ km/h.)
\[\boxed{9\text{ km/h}}\]