For the First case

[ As time = distance / speed ]

[ Also let velocity of boat be 'b' and that of the stream be 's' ]

(24/(b-s))+(36/(b+s))=6 ----1

and for the second case

(36/(b-s))+(24/(b+s))=6.5 ---2

Now from 1 - 2

and 1 + 2

we can easily get

b/s=5/1

ie., b=5s ( boat's speed is 5 times greater than that of the stream ) ---3

substituting 3 in (1 - 2)

yields s= 2 km/hr.