A launch moves at 12 km/h in still water, and the stream flows at 3 km/h. How long does it take to go 45 km downstream and come back?

7.5 hoursCorrect answerYour answer8 hoursCorrect answerYour answer9 hoursCorrect answerYour answer10 hoursCorrect answerYour answer

Show the answer and explanation

CorrectNot quiteAnswer B

Correct answer: B

Down at 15 km/h: $\frac{45}{15} = 3$ hours. Back at 9 km/h: $\frac{45}{9} = 5$ hours.
Total 8 hours.

Worth noting

7.5 hours treats the water as still; the slow leg upstream costs more than the fast leg saves.

Ten questions from Boats & Streams, Pipes & Cisterns, at your own pace.

Ten questions from Boats & Streams, Pipes & Cisterns, at your own pace.

Explanation