Multiple choice

The speed of a boat in still water is 15 km/hr. If it can travel 42 km downstream and 28 km upstream in the same time, then what is the speed of the stream?

  1. 2.5 km/hr

  2. 3 km/hr

  3. 4.5 km/hr

  4. 6 km/hr
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let stream speed be s. Downstream speed = 15+s, upstream = 15-s. Time is equal: 42/(15+s) = 28/(15-s). 42(15-s) = 28(15+s). 3(15-s) = 2(15+s). 45-3s = 30+2s. 15 = 5s. s = 3 km/hr.

AI explanation

Let the speed of the stream be x km/hr, making the downstream speed (15 + x) and the upstream speed (15 - x). Using the equality of time for both trips, 42 divided by (15 + x) equals 28 divided by (15 - x). Cross-multiplying yields 42 multiplied by (15 - x) = 28 multiplied by (15 + x), which simplifies to 630 - 42x = 420 + 28x. Solving this equation gives 70x = 210, so x = 3 km/hr.

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