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Question
A boat takes 4 hours to travel from a place X to Y downstream and back from Y to X upstream. If the distance from X to Y is 10.5 km, and the speed of the current is 9 km/h, then the speed of the boat in still water, in km/h, is:
Options
`10 1/2`
15
12
`12 1/2`
MCQ
Solution
12
Explanation:
`10.5/(x + 9) + 10.5/(x - 9)` = 4
[Put your option to solve for 'x']
`10.5 [(x - 9 + x + 9)/(x^2 - 81)]` = 4
`21x = 4x^2 - 324`
`4x^2 - 21x - 324` = 0
`x = 12, (-27)/4`
This is not possible.
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Boats and Streams (Entrance Exam)
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