Question

The time taken by a person to cover a certain distance on a motorboat when travelling downstream is 4 hours, while the same distance is covered in 5 hours upstream. If the speed of the stream is 2 kmph, then find the speed of the motorboat in still water.

• A. 18 kmph
• B. 20 kmph
• C. 16 kmph
• D. 14 kmph

Answer 1

The correct option is A 18 kmph

Let the speed of the motorboat in still water speed be $x$ kmph.

Speed of boat downstream $=(x+2)$ kmph.
Distance covered in 4 hours $=4(x+2)$ km.

Speed of boat upstream $=(x-2)$ kmph.
Distance covered in 5 hours $=5(x-2)$ km.

The distance covered is the same.
Hence,
$\Rightarrow 4(x+2)=5(x-2)\Rightarrow 4x+8=5x-10\Rightarrow 8+10=5x-4x\Rightarrow x=18$

Therefore, the speed of the motorboat in still water is 18 kmph.