A boat can cover 20 km upstream and 32 km downstream together in 3 hours Also it can cover 40 km ups...........

Explanation:

Upstream speed in both cases is 20 and 20 resp. Ratio is 20 : 40 = 1 : 2.
So let times in both cases be x and 2x
Downstream speed in both cases is 32 and 48 resp. Ratio is 32 : 48 = 2 : 3.
So let times in both cases be 2y and 3y
So x + 2y = 3
and 2x + 3y = 5 1/2
Solve both, x = 2, y = 0.5
So upstream speed is = 20/x = 10 km/hr
And downstream = 32/2y = 32 km/hr
So speed of boat is 1/2 * (32-10) = 11 km/hr