Question
Motor boat covers a certain distance downstream in 5
hours. It covers the same distance upstream in 7(1/2) hour. If the speed of the water is 4.5 km/hr, then the speed of the boat in still water is:Solution
Let the distance be ‘D’ km According to the question, Downstream speed x + y = d/5 or d = 5 (x+y) .......... (i) Upstream speed x-y = (d×2)/15 or d = (15(x-y))/2 ........... (ii) Comparing (i) & (ii) 5 (x+y) = (15(x-y))/2  10(x+y) = 15(x-y) 2(x+y) = 3(x-y) 2x + 2y = 3x - 3x x = 5y Hence, y = Speed of the current = 4.5 km/hour. Speed of boat, x = 5 × 4.5 = 22.5 km/hr
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