Step 1:
Let the rate(speed) of motor boat = m
Let the rate(speed) of current = n
Step 2:
Distance = Speed X Time
Time = 18hour for down a river
Total distance for flowing downward = 135
Total distance for flowing upstream due to current = 135
Step 3:
Add the first and the second equation.
Substitute m in any equation to find n.
Final answer
Rate of the motor boat in still water m = 6 miles/hr
Rate of the current n = 1.5 miles/hr