A man can row 4 kilometers per hour in still water. When the river is flooded, he found out that it requires 6 1/2 hours to row upstream, a distance requiring 3 1/2 hours to row downstream. What is the rate of the current?

Answer 1

let the rate of the current be x km/h

going upstream:
distance = 6.5(4 -x)

going downstream:
distance = 3.5(4 + x)

but 3.5(4+x) = 6.5(4-x)
multiply by 2 to get whole numbers
7(4+x) = 13(4-x)
28 + 7x = 52 - 13x
20x = 24
x = 24/20 = 1.2

the current has a rate of 1.2 km/h