Question

a motorboat goesdownstream and covers the distance between two ports in 5hours and return back in in 7hours if the water is flowing at 2km/h find the speed of the boat in still water

Answer 1

Let speed of boat in still water be X km/hr
So speed of water in the river is X + 2 km/hr downstream
And  it is                                      X - 2 km/hr upstream
Let Y be the distance to port
Now as speed = distance / time ,
time = distance / speed 
5 hrs = Y / X+2 ........ (1)
7 hrs = Y / X-2 .........(2)
hence from 1 and 2 , 
Y = 5x +10
Y = 7x - 14
Subtracting both
0= 5x-7x+10+14  ( - of - = +)
0= -2x +24
hence 
2x= 24
x= 12 km/hr
hence speed of boat in still water is 12 km/hr.