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:

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 hour
= 3/2 hour

⇒ 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 − x²)
⇒ 48 x = 100 − x²
⇒ x² + 48x − 100 = 0
⇒ (x + 50) (x − 2) = 0
⇒ x = 2 (ignoring -ve value)