Question

Motor boat covers a certain distance downstream in a river in 20 hours. It covers the same distance upstream in 30 hours. If the speed of the water is 12 km/hr, then the speed of the boat in still water is:

A 55 km/hr Correct Answer Incorrect Answer
B 60 km/hr Correct Answer Incorrect Answer
C 50 km/hr Correct Answer Incorrect Answer
D 75 km/hr Correct Answer Incorrect Answer
E 90 km/hr Correct Answer Incorrect Answer

Solution

Let the distance be ‘D’ km According to the question, Downstream speed  x + y = d/20 or d = 20 (x+y) .......... (i) Upstream speed x-y = d/30 or d = 30(x-y) ........... (ii) Comparing (i) & (ii) 20(x+y) = 30(x-y) 2(x+y) = 3(x-y) 2x + 2y = 3x - 3y x = 5y Hence, y = Speed of the current = 12 km/hour. Speed of boat, x = 5 × 12 = 60 km/hr

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