Question 161585
We know that the ratio of the distance travel is 3:1     36:12

We then make the distance equal by dividing the distance traveled downstream by 3
We know the speed of the river=5 km/h

x=time it takes to travel upstream
9-x time it takes to travel downstream

5x=1/3(9-x)(5)

This equation is based on D=(rate)(time)

Muliply both sides by 3

15x=(9-x)(5)

Use the distributive property

15x=45-5x

add 5x to both sides

20x=45

divide both sides by 20 x=2.25  9-x=6.75

Lets check the ratio to make sure that it is 3:1

6.75/2.25=3

Let's plug into the D=TR to calculate rate

36/6.75=5.33 km/hr

12/2.25=5.33 km/hr

The rate is the same.