Multiple choice

The speed of a boat in still water is 8 km/hr. It can travel 20 km downstream at the same time as it can travel 12 km upstream, the rate of stream (in kmph) is

  1. 0.5

  2. 2

  3. 2.5

  4. 2.75
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let the stream speed be x. Downstream speed = 8 + x, upstream speed = 8 - x. Time = Distance / Speed. 20 / (8 + x) = 12 / (8 - x). Solving: 20(8 - x) = 12(8 + x) => 160 - 20x = 96 + 12x => 64 = 32x => x = 2.

AI explanation

Let the rate of the stream be x kmph; the downstream speed is 8 + x and the upstream speed is 8 - x. Because the times are equal, 20 divided by (8 + x) equals 12 divided by (8 - x). Cross-multiplying yields 160 minus 20x equals 96 plus 12x, which simplifies to 64 equals 32x, meaning x is 2 kmph.

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