This is a great riddle to get people thinking, because it's not an obvious answer!
River A, you have: a fox, a goose, and a bag of corn.
So, you've probably gone through this looking for some way to bring one of them over, but if she takes the cornmeal, the fox will eat the goose, if she takes the fox, the goose will eat the corn, and if she takes the goose, then the fox and the cornmeal will be left together (which isn't bad until you have to pick one of them to take over with the goose).
Let's start by picking one of the animals (or objects) to take over first. Because it takes two trips for things to get bad with the goose, will do it first.
We take the goose across river A, and come back to the fox and the cornmeal. Nothing bad has happened. Now we know that if we leave the goose with the fox or the cornmeal, the cornmeal or the goose will be eaten.
There is one other thing we can do! The girl can only take one thing back and forth, but she can bring something back when she takes one of them across!
This means that:
She can take the goose first and the cornmeal second, but when she takes the cormeal, she can grab the goose and bring it back with her, so now the cornmeal is across river A but the goose isn't! Then, she can drop off the gosse and grab the fox, because the fox won't do anything to the cornmeal, and then come back for the goose!
Just ask if you have an questions! :)