There is point A in the upstream and point B in the downstream of the river of 6km per an hour at the speed of the flow.
It takes three hours for the ship to go up from B to A and two hours go down from A to B.
How much is the distance between A and B?
Answer
72 km
Solution
Because going up and down in the same distance, the ratio of the time to take and the ratio of the speed become the inverse proportion.
Because the ratio of the time going up and down is 3:2, the ratio of the speed going up and down is = 2 : 3.
The speed of the flow of the river is (3-2)/2 = 0.5 which is described by the proportion and 0.5 correspond to 6km per hour which is actual speed of the river flow.
As the speed of going up is 2 which correspond to 24km per hour by the calculation of (6/0.5) x 2 = 24.
As it took three hours to go up, the distance between A and B is 24 x 3 = 72 km.