A boat goes 24 km upstream and 28 km downstream in 6 hours. It goes 30km upstream and 21km downtream in 6.5 hours. Find the speed of the boat in the still water and the speed of stream.

Let speed of boat be x kmph and of the stream be y kmph

When boat goes upstream, effective speed of boat will be x - y and when boat goes downstream, its speed will be x+y.

Case 1 : 

To travel 24 km upstream, it takes 24/(x-y) hrs

To travel 28 km downstream, it takes 28 / ( x + y ) hrs

 24/(x-y)   +  28 / ( x + y )  = 6   -  - -  ( 1 ) 

Similarly for case 2, we get

30/(x-y)   +  21 / ( x + y )  = 6.5     -  - -  ( 2) 

Solve equations 1 and 2