Lets say we have 7 items, and we add 7 more.
Then, in the next situation, we have 7 items and we add 8 more. Now we know that 8 = 7 + 1, so we are actually performing 7 + 7 +1. Our diagram already shows us the value of 7 + 7 so we just add 1.
The items can be represented by circles, triangles etc