Question:If a boat goes `7` km upstream in `42` minutes and the speed of the stream is `3` kmph, then the speed of the boat in still water is : 

A `4.2 km/hr` 
B `9 km/hr` 
C `13 km/hr` 
D `21 km/hr` 

+ Answer
C 

+ Explanation
Rate upstream 

`= (7/42 xx 60) kmph`

`= 10 kmph`

Speed of stream `= 3 kmph`

Let speed in still water be x km/hr

Then, speed upstream 

`= (x - 3) km/hr`

`:. x - 3 = 10`

` or x = 13 kmph`

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Irin Moni

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