algebra
posted by shelby .
a motorboat can maintain a constant speed of 11 miles per hour relative to the water. The boat makes a trip upstream to a certain point in 21 minutes; the return trip takes 1 minutes. That is the speed of the current?

PsyDAG answered the same question here
http://www.jiskha.com/display.cgi?id=1276112108
Just change the numbers to fit your problem.