Multiple choice

A boat takes two hours to travel 8 km down and 8 km up the river when the water is still. How much time will the boat take to make the same trip when the river starts flowing at 4 kmph?

  1. 2 hour

  2. 2 hour 40 minute

  3. 3 hour

  4. 3 hour 40 minute
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

In still water, the boat takes 2 hours for a total of 16 km, meaning its speed is 8 km/h. When the river flows at 4 km/h, the downstream speed is 12 km/h and the upstream speed is 4 km/h. The total time for the trip is 8/12 + 8/4 = 2/3 + 2 = 2 hours and 40 minutes.

AI explanation

In still water, the boat covers 16 km in 2 hours, giving a boat speed of 8 kmph. With a stream speed of 4 kmph, the downstream speed becomes 12 kmph and the upstream speed becomes 4 kmph. The time for the 8 km downstream leg is 8 divided by 12, which is 0.66 hours, and the time for the 8 km upstream leg is 8 divided by 4, which is 2 hours. The total time is 2.66 hours, equal to 2 hours 40 minutes.

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