Multiple choice

A river is flowing at a steady speed of 3 km/hr. P starts swimming from one bank to the other bank of the river and comes back to the initial position. Find the distance travelled by P in the whole journey, if his speed in still water is 9 km/hr and he takes 2 hr to complete the whole journey.

  1. 16 km

  2. 24 km

  3. 12 km

  4. 36 km

  5. 18 km
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A Correct answer
Explanation

Speed downstream = 9 + 3 = 12 km/hr. Speed upstream = 9 - 3 = 6 km/hr. Let distance be d. d/12 + d/6 = 2. (d + 2d)/12 = 2, 3d = 24, d = 8 km. Total distance = 2d = 16 km.

AI explanation

P's downstream speed is his still water speed plus the stream speed, giving 9 + 3 = 12 km/hr, and his upstream speed is 9 - 3 = 6 km/hr. Let the width of the river be W kilometers, so the total time is W/12 + W/6 = 2 hours. Solving this equation gives 3W/12 = 2, meaning W is 8 kilometers. Since he travels to the other bank and back, the total distance travelled is 8 + 8 = 16 km.

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