Rowing at the rate x miles per hour down the river, the rower covers the distance   miles.

    Notice   =  of an hour = 20 minutes.



On the way back, the rower's rate is (x-1) miles per hour, so his way back is    miles long.

    Notice   =  of an hour = 30 minutes.


The distance both ways is the same


     = .


Solve it. Start multiplying both sides by 6. You will get


    2x = 3*(x-1)

    2x = 3x - 3

    3 = 3x - 2x

    x = 3.


Thus you found the rate of canoe downstream:  it is 3 miles per hour.


In 20 minutes, it gives the distance   = 1 mile.    ANSWER


You can check the answer by your own by substituting the found values into the condition.