Imagine you are on a main street which splits to two smaller streets, one with three shops and the other with five shops. If all these shops were different, you would know that you have \(3+5=8\) different shops you can go to.
Moreover, if some shops are connected to both smaller streets, you still know that you have \(3+5=8\) different choices you can make (i.e. pathways you can take followed by the shop you choose), even if the number of shops is less than eight.
One important thing you do know is that you can’t be on both streets at the same time, by common sense. From this you know that if there are \(3\) ways to go from street 1 and \(5\) ways to go from street 2, you have \(5+3=8\) ways to go, if you choose to go from “street 1 or street 2”.