let

v = the speed of the boat in still water, v>0

c = 3 mph the speed of a river current

d1 = 30 mi traveling downstream

d2 = 20 mi traveling upstream

traveling downstream the speed of the boat is: v + c

traveling upstream the speed of the boat is: v - c

since speed = distance/time we have time = distance/speed we have

d1/(v + c) = d2/(v - c)

30/(v + 3) = 20/(v - 3)

by solving we find:

v = 15 mph

click here to see the step by step solution of the equation:

the speed of the boat in still water is 15 miles per hour.