The speed of a boat in still water is 15 km/h and the speed of the stream is 3 km/h. How long will the boat take to travel 54 km downstream?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 3 hours
Going downstream, the stream adds its speed to the speed of the boat:
\(\text{Downstream speed} = 15 + 3 = 18\) km/h
\(\text{Time} = 54 \div 18 = 3\) hours
Why the other options are incorrect
Option (1) 2.5 hours — Incorrect. In 2.5 hours the boat covers only \(18 \times 2.5 = 45\) km downstream.
Option (3) 3.5 hours — Incorrect. In 3.5 hours the boat would cover \(18 \times 3.5 = 63\) km downstream, more than 54 km.
Option (4) 4.5 hours — Incorrect. This is the time for 54 km upstream, at \(15 - 3 = 12\) km/h.