Multiple choice

A man can row a boat with $4 km/h$ in still water. If he is crossing a river where the current is $2 km/h$ and width of river is $4 km$. How long will it take him to row $2 km$ up the stream and then back to his starting point?

  1. $2 hrs$
  2. $1 hr$
  3. $\dfrac{4}{3} hrs$

  4. None of the above
Reveal answer Fill a bubble to check yourself
C Correct answer
Explanation

Upstream speed = 4 - 2 = 2 km/h. Downstream speed = 4 + 2 = 6 km/h. Time upstream = 2 km / 2 km/h = 1 hour. Time downstream = 2 km / 6 km/h = 1/3 hour. Total time = 1 + 1/3 = 4/3 hours.

AI explanation

The speed of the boat in still water is 4 km/hr and the stream speed is 2 km/hr. Using the boat and stream formula, the upstream speed is 4 - 2 = 2 km/hr and the downstream speed is 4 + 2 = 6 km/hr. The time to row 2 km upstream is 2 divided by 2, which is 1 hour, and the time to return 2 km downstream is 2 divided by 6, which is 1/3 of an hour. Adding these times gives 1 + 1/3 = 4/3 hours.

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