If 1/3 of the students got an A. And 2/5 got a B, what fraction of the students got an A or a B

You have to add the two fractions together, but you can't because the denominators are different. You have to get the denominators to be the same in order to add them. Something that both 3 and 5 go into is 15:

5/1 * 1/3 = 5/15

5?1 * 2/5 = 6/15

5/15 + 6/15 = 11/15

Answer: 11/15