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 choice from the following answers.
A) 4 mph
B) 2.5 mph
C) 3 mph
D) 2 mph
E) None of these
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   =&gt;36/(10−x) − 36/(10+x) = 3/2   =&gt;12/(10−x) − 12/(10+x) = 1/2   =&gt;24(10 + x) − 24(10 − x) = (10 + x)(10 − x)   =&gt;240 + 24x − 240 + 24x = (100−x2)   =&gt;48x = 100 − x2   =&gt;x2 + 48x − 100 =0   =&gt;(x+50)(x−2)=0   =&gt; x = -50 or 2. Answer : Option 2 mph
<result>D</result>
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: