Seven add eight equals what?
Here we have two ten frames, one with seven counters and another with eight. In the tens frame with seven counters, there are three empty spaces. We could take three counters from our other ten frame and add them to the seven to make 10: one, two. Now we have 10 counters in this ten frame and five counters left in the other ten frame. 10 add five equals 15.
We started with seven and eight. We split the eight counters into two parts: three and five. First, we added three to seven. Seven and three are a pair that make 10. And then we added five. 10 and five is 15. Seven add eight equals 15.