Boat and Hat problem.

Algebra Level 3

Point A and B are 1 km apart. Mike starts rowing from A towards B, along a river that flows at a constant rate. As he passes B his Hat falls into the river. Ten minutes later, he notices his hat and turns the boat around instantaneously rowing at the same constant rate. By the time he reaches the hat, he is at A.

How fast is the river flowing?

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