Aaron is making 3 1/2 batches of brownies. each batch takes 2 1/2 cups of flour. how much flour will john need for the brownies?

Here in this problem your adding. To do that start with aligning the problem vertically. Like this
3 1/2 now you add the halves which equal a
+ 2 1/2 whole. So now you add 3+2+1 which equals 6. 6 is the complete answer.

Yes, two vertical lines can be complementary