Explanation:

Solution: 
Upstream speed in both cases is 14 and 21 resp. Ratio is 14 : 21 = 2 : 3. So let times in both cases be 2x and 3x
Downstream speed in both cases is 21 and 42 resp. Ratio is 21 : 42 = 1 : 2. So let times in both cases be y and 2y
So 2x + y = 3
and 3x + 2y = 5
Solve both, x = 1, y = 1
So upstream speed is = 14/2x = 7 km/hr
And downstream = 21/y = 21 km/hr
So speed of current is 1/2 * (21-7) = 7 km/hr