Explanation:

Let the speed of the stream x mph.

Then,
Speed Downstream = (10 + x) mph,

Speed Upstream = (10 - x) mph.

Therefore,36 /(10-x) - 36 / (10+x) = 90/60 

=> 72x x 60 = 90 (100 - x2)

=> x2 + 48x - 100 = 0

=> (x+ 50)(x - 2) = 0

=> x = 2 mph.