maths problem

Hi there!
I have a math problem I found on the internet (or maybe it's more physics, I dunno). I have a solution, but as nobody put up the correct answer, I don't know if I have it right.
So if anybody has the time and interest, here's the question, thanks for posting your answers

The Harvard bridge and the Longfellow bridge cross the Charles River in a distance of 1 mile from each other. A boat team is rowing upstream from the longfellow bridge. As the boat passes the Harvard bridge, the helmsman's hat falls into the river. After ten minutes he notices and turns around the boat immediately, letting the team row downstream with the same amount of beats per minute. When they reach the hat, they are at the Longfellow bridge again.

What speed is the river flowing at?

3 miles/hour.

I also got 3 miles/hour, and found that the speed of the boat is not determinable (not that it matters, since you didn't ask for it).

Just take the time required for the boat to reach the Longfellow bridge and set it equal to the time required for the hat to reach it:

s = speed of boat
c = current (speed of river)

[1 + (s - c)/6]/(s + c) = (1 - c/6)/c
c + sc/6 - (c^2)/6 = s + c - sc/6 - (c^2)/6
sc/6 = s - sc/6
sc/3 = s
c/3 = 1
c = 3

Originally posted by Jirakon
[b]I also got 3 miles/hour, and found that the speed of the boat is not determinable (not that it matters, since you didn't ask for it).

Yes, the speed of the boat does not matter. It can be any speed greater than 3 miles an hour. It has to be greater than 3 miles an hour, of course, otherwise it would not have been able to travel upriver.