A dray moves at the speed of 30 m/m to B point 6000 m away from A point.
When a dray moves 900 m, Taro in A point start to pursue the dray with a ball.
If he catches up with the dray, he put the ball in the dray and he will return toward A point immediately.
He receives a ball at A point and start to pursue the dray again.
Taro repeats this motion until the dray arrives at B point.
The speed of Taro is 90 m/m.
Since Taro begins to move, find the time until he finally puts a ball in the dray since he begins to move at first.