I am having trouble solving this particular question that was on my test and I keep on getting the same result.

This is the problem:

~A women who can row 3 miles per hour in still water rows 10 miles downstream on the Kern river and then turns around and rows back upstream to her starting point. Her total trip took 12 hours. Find the speed of the current.~~

I formed the equation and I thought it was supposed to look like this:

3 3
----- + --------- = 12
10-x 10+ x

My answer was 13.....unfortunately my teacher marked that as wrong. So what did I do wrong? Did I wrote the equation wrong? So what is the answer then and what is the correct equation for this problem??? Please help!

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