Multiple choice

A boat goes 20 km downstream in one hour and the same distance upstream in two hours. The speed of the boat in still water is

  1. 15 km/hr

  2. 10 km/hr

  3. 5 km/hr

  4. 7.5 km/hr
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A Correct answer
Explanation

Let v be the speed of the boat and c be the speed of the current. Downstream speed is v + c = 20/1 = 20 km/hr. Upstream speed is v - c = 20/2 = 10 km/hr. Adding these equations gives 2v = 30, so v = 15 km/hr.

AI explanation

Using the basic Time, Speed and Distance formula, the downstream speed is 20 divided by 1, which is 20 km/hr, and the upstream speed is 20 divided by 2, which is 10 km/hr. The speed of the boat in still water is the average of its downstream and upstream speeds. This gives (20 + 10) divided by 2, which equals 15 km/hr.

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