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The speed of the current is 5 km/h. A motorboat goes 10 km upstream and back again to the starting point in 50 min. The speed, (in km/h) of the motorboat in still water is ?
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- 20 km/h
- 26 km/h
- 25 km/h
- 28 km/h
Correct Option: C
Let speed of boat be x km/h.
Given speed of current = 5 km/h
∴ Upstream speed of boat = (x -5) km/h
Downstream speed of boat = (x + 5) km/h
According to the question.
10/(x - 5) + 10/(x + 5) = 50/60
⇒ 10(x + 5 + x - 5)/( x2 - 25) = 5/6
⇒ x2 -24x - 25 = 0
⇒ x2 - 25x + x - 25 = 0
⇒ (x - 25) (x + 1) = 0
∴ x = 25 [∴ x ≠ -1]
