Aboat's crew rowed 13.5 miles downstream, with the current, in 1.5 hours. the return trip upstream, against the current, covered the same distance, but took 4.5 hours. find the crew's rowing rate in still water and the rate of the current.

V - the crew rowing rate,
u - rate of the current.
Downstream:  v + u = 13.5/1.5 = 9 mph
Upstream :      v - u = 13.5/4.5 = 3 mph
u = 9 - v
v - ( 9 - v ) = 3
v - 9 + v = 3
2 v = 12
v = 6 mph,   u = 9 - 6 = 3 mph
the crew`s rowing rate is 6 mph and the rate of the current is 3 mph.