Multiple choice

A man swimming in a river, which flows at 1 1 2 km/hr, finds that in a given time, he can swim twice as far along the stream as he can against it. At what speed does he swim in still water?

  1. 5 1 2 km/hr

  2. 4 1 2 km/hr

  3. 7 1 2 km/hr

  4. None of these
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let the swimmer's speed in still water be v km/hr and the stream speed be 1.5 km/hr. Since downstream distance is twice upstream distance in the same time, (v + 1.5)/(v - 1.5) = 2, giving v = 4.5 km/hr.

AI explanation

Let the speed of the man in still water be x km/hr. Since the river flows at 1.5 km/hr, his downstream speed is (x + 1.5) km/hr and upstream speed is (x - 1.5) km/hr. Because the time is constant, the ratio of distances equals the ratio of speeds, giving (x + 1.5) / (x - 1.5) = 2. Solving this, x + 1.5 = 2x - 3, which means x = 4.5 km/hr.

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