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 :	Choose the correct choice from the following:  A) 4.2 km/hr  B) 9 km/hr  C) 13 km/hr  D) 21 km/hr  E) None of these
Solution   Rate upstream =(7/42x60)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 km/hr. Answer 13 km/hr
<result>C</result>
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 :