Multiple choice

A boat goes 42 km upstream and comes back to the starting point in 306 minutes. If stream running at 3 kmph then, what is the speed of the boat in still water?

  1. 15 kmph

  2. 17 kmph

  3. 19 kmph

  4. 22 kmph

  5. 25 kmph
Reveal answer Fill a bubble to check yourself
B Correct answer
Explanation

Let boat speed = b kmph. Speed upstream = b - 3, downstream = b + 3. Total time = 42/(b-3) + 42/(b+3) = 306/60 = 5.1 hours. Solving: 42(1/(b-3) + 1/(b+3)) = 5.1. After cross-multiplication: 42(b+3 + b-3)/((b-3)(b+3)) = 5.1. This gives 84b/(b²-9) = 5.1, or 84b = 5.1b² - 45.9. Rearranging: 5.1b² - 84b - 45.9 = 0. Using quadratic formula: b = (84 ± √(7056 + 936.36))/10.2 = (84 ± 89.4)/10.2. Taking positive root: b = 17 kmph. Option B is correct. Options A, C, D, E are computational errors.

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