Multiple choice

A boat can go 3.6 km upstream and 5.4 km downstream in 54 minutes, while it can go 5.4 km upstream and 3.6 km downstream in 58.5 minutes. The time (in minutes) taken by the boat in going 10 km downstream is:

  1. 54

  2. 48

  3. 50

  4. 45
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Let 1/u = x and 1/d = y where u, d are upstream/downstream speeds. From the trips: 3.6x + 5.4y = 0.9 and 5.4x + 3.6y = 0.975. Solving gives y = 1/12, so d = 12 km/h. For 10 km downstream: time = 10/12 hours = 50 minutes. Options A, B, and D are computational errors.

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