A this usual rowing rate Mohit can travel 12 miles downstream in a certain river in 6 hours less tha...........

Quantitative Aptitude ( CCS) Boats and Streams

A this usual rowing rate, Mohit can travel 12 miles downstream in a certain river in 6 hours less than it takes him to travel the same distance upstream. But if he could double his usual rowing rate for his 24 miles round trip, the downstream 12 miles would then take only one hour less than the upstream 12 miles. What is the speed of the current in miles per hour?

2.5m/hr

4 m/hr

2 (2/3) m/hr

5/3m/hr

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Explanation:

Speed of the stream = ½ (a-b) km/hr
Explanation:
Let the speed in still water be x m/hr
Speed of stream be y m/hr
Then, speed upstream = x-y m/hr and
Speed downstream = x+y m/hr
12/x-y – 12 / x+y = 6 so,6 (x^2 – y^2) = 24 y
x^2 – y^2 = 4y
x^2 = y^2 + 4y…………..1
also
12/ 2x-y – 12/2x +y = 1 4x^2 – y^2 = 24y
x^2 = [24y + y^2] / 4 ……………….2
16y + 4y^2 = 24y + y2 [put X^2 value from 1] 3y^2 = 8 y so, y = 8/3
speed of the current = 8/3 m/hr = 2 (2/3) m/h

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