A boat takes 90 minutes less to travel 36 miles downstream than to travel the same distance upstream. If the speed of the boat in still water is 10 mph, the speed of the stream is:  Choose the correct option: A) 4 mph B) 2.5 mph C) 3 mph D) 2 mph E) none of these

Rationale: explanation : speed of the boat in still water = 10 mph let speed of the stream be x mph then , speed downstream = ( 10 + x ) mph speed upstream = ( 10 - x ) mph time taken to travel 36 miles upstream - time taken to travel 36 miles downstream = 90 / 60 hours = > 36 / ( 10 − x ) − 36 / ( 10 + x ) = 3 / 2 = > 12 / ( 10 − x ) − 12 / ( 10 + x ) = 1 / 2 = > 24 ( 10 + x ) − 24 ( 10 − x ) = ( 10 + x ) ( 10 − x ) = > 240 + 24 x − 240 + 24 x = ( 100 − x 2 ) = > 48 x = 100 − x 2 = > x 2 + 48 x − 100 = 0 = > ( x + 50 ) ( x − 2 ) = 0 = > x = - 50 or 2 . answer : option d
Answer: D