Multiple choice

A boat takes $4$ hours to move $10\ km$ and back to starting point in still water. If the river flow velocity is $2\ km/hr$, then time taken by the boat to move $10\ km$ upstream and back to starting point is approximately

  1. $4\ hours$
  2. $5\ hours$
  3. $6\ hours$
  4. $7\ hours$
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In still water, the boat travels 10 km and back in 4 hours, so speed in still water (v) is 10 / 2 = 5 km/h. With stream speed (u) = 2 km/h, upstream speed is 5-2 = 3 km/h and downstream is 5+2 = 7 km/h. Total time = 10/3 + 10/7 = 3.33 + 1.43 = 4.76 hours, which is approximately 5 hours.

AI explanation

Using the formula time equals distance divided by speed, the boat's speed in still water is 20 km divided by 4 hours, giving 5 km per hour. The downstream speed is 5 plus 2, yielding 7 km per hour, and the upstream speed is 5 minus 2, yielding 3 km per hour. The total time to travel 10 km downstream and 10 km upstream is 10 divided by 7 plus 10 divided by 3, which is roughly 4.76 hours, therefore the time taken is approximately 5 hours.

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