Question
A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream . If the speed of the boat in still water is 10 mph, the speed of the stream is :
Answer: Option A
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Let he speed of he stream be `x` mph, Then
Speed downstream = `(10 + x)` mph ,
Speed upstream = `(10 - x)` mph
`:.` `36/(10 - x) - 36/(10 + x) = 90/60`
`hArr 72x xx 60 = 90(100 -x^2)`
`hArr x^2 + 48 x + 100 = 0`
`hArr (x + 50) (x - 2) = 0 hArr x` = 2 mph.
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