Question 5684
You are so in the right track. Your setup is almost all right. I'm thinking about the upstream rate, though. You put x - 9 for the upstream rate. I think it would be 9 - x. Say what!?


We agree that all speeds in a forward direction must be positive, right? If x - 9 was positive, that would force x (the rate of the stream) to be greater than 9, which, if you're moving upstream, won't let you move forward . (In fact, you'll actually be moving backwards). If she was able to move upstream, then obviously, the speed of the current had to have been less than the speed of her boat in still water. The way to have the current's speed less than the boat's speed in still water, and for the upstream net speed to be positive, the upstream rate would have to be 9 - x.


I hope that helped. You were so close. I'd say you did a good job, getting this far (setting up the problem) takes a lot.


Here's your equation:


{{{ 12/(9-x) + 12/(9+x) = 3 }}} Solve for x and you'll get the current's speed.