Explanation:

Let the speed of the stream x mph. Then,

Speed downstream = (10 + x) mph,

Speed upstream = (10 - x) mph

    36             36           
----------  -  ---------    =   90/60   ---------> (i)
(10 - x)     (10 + x)       

72x * 60 = 90 (100 - x²)
x² + 48x - 100 = 0
(x+ 50)(x - 2) = 0
x = 2 mph

Putting up the options in equ (i) is a better idea in the cases where we get Quadratic equations.