The speed of the current is 5 km hour A motorboat goes 10 km upstream and back again to the starting...........

Quantitative Aptitude(IBPS PO) Boats and Streams

The speed of the current is 5 km/ hour. A motorboat goes 10 km upstream and back again to the starting point in 50 minutes. The speed (in km/hour) of the motorboat in still water is

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Explanation:

Speed of the current y = 5 km/h
Let the speed of the motor boat in still water = x km/h
10/(x + 5) + 10/(x - 5) = 5/6
Take option (c) 10/(25 + 5) + 10/(25 - 5) = 10/30 + 10/20
= 1/3 + ½ = 5/6
Thus, Option c is the answer.

Alternate:
T = 2xD/(x² – y²)
50/60 = 2 × x × 10/(x² - 5²)
5/6 = 20x / (x² – 25)
x²– 24x – 25 = 0
x = 25, – 1
x = 25 km/h