<PRE><B>Question 1071860</B>
current is 4 mph
speed of boat =x
downstream, speed is 4+x and time is 20/x+4, because time is distance over speed.
upstream, speed is 4-x and time is 10/x-4
The two times are equal
so 20/x+4=10/x-4
20x-80=10x+40
10x=120
x=12 mph
16 mph downstream and takes 1h15m to go 20 miles (1.25 hr)
8 mph upstream and takes 1h15m to go 10 miles.</PRE>