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Question
The speed of a boat in still water is 15 km/h and speed of stream is 5 km/h. The boat goes x km downstream and then returns back to the point of start is ______.
Options
`(x/20 - x/5)` hrs
`(x/10 - x/20)` hrs
`(x/20 + x/10)` hrs
`(x/20 - x/10)` hrs
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Solution
The speed of a boat in still water is 15 km/h and speed of stream is 5 km/h. The boat goes x km downstream and then returns back to the point of start is `underlinebb((x/20 + x/10) hrs)`.
Explanation:
Given the speed of boat in still water = 15 km/h
Speed of stream = 5 km/h
∴ Speed of boat upstream
= 15 – 5
= 10 km/h
And the speed of boat downstream
= 15 + 5
= 20 km/h
∴ Time is taken by boat x km downstream
@ 20 km/h = `"Distance"/"Speed"` = `x/20` hrs
Time is taken by boat x km upstream
@ 10 km/h = `x/10` hrs
Thus, the total time taken by boat goes x km downstream and then return to the point of start = `(x/20 + x/10)` hrs.
