Multiple choice

A man goes 48 km downstream and 20 km upstream. If he takes 4 hours to cover each distance, then what is the speed of the current?

  1. 3 km/h

  2. 8 km/h

  3. 8.5 km/h

  4. 3.5 km/h

  5. None of these
Reveal answer Fill a bubble to check yourself
D Correct answer
Explanation

For downstream: speed = boat speed + current speed. Distance = 48 km, time = 4 hrs, so downstream speed = 48/4 = 12 km/h. For upstream: speed = boat speed - current speed. Distance = 20 km, time = 4 hrs, so upstream speed = 20/4 = 5 km/h. Adding: (boat + current) + (boat - current) = 12 + 5 = 17, giving 2 × boat speed = 17, so boat speed = 8.5 km/h. Subtracting: (boat + current) - (boat - current) = 12 - 5 = 7, giving 2 × current speed = 7, so current speed = 3.5 km/h.

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