60-20 = 40m (countercurrent speed)

The speed ratio of downstream and upstream = 80: 40 = 2: 1.

Time ratio between downstream and upstream = 1: 2

3: 30 = 2 10

2 10/( 1+2) = 70 minutes (water use time)

70 * 80 = 5600 meters (a * * * distance)