A boat takes 3.0 h to travel 36 km down a river, then 5.0 h to return. How fast is the river flowing? Express your answer in kilometers per hour.
Let the speed of the boat be x km/h
and speed of the river be y km/h
when it travels down the river
speed = distance/ time
(x + y) = 36 / 3
x + y = 12 ----------(1)
when it tavels up stream
(x - y) = 36 / 5
x - y = 7.2 --------(2)
adding both equations we have
2x = 19.2
x = 9.6 km/h
y = 12 - x
= 12-9.6 = 2.4 km/h
so the speed of the river is 2.4 km/h