Answer:
So we have the addition.
7 + 2 + 6.
First, she added the 7 + 2 = 9
So we can replace that in our addition as:
7 + 2 + 6 = (7 + 2) + 6 = 9 + 6
Now, we can use 9 = 5 + 4, and replace that in the addition.
Then we have:
9 + 6 = (5 + 4) + 6
And we know that 4 + 6 = 10.
Then:
5 + 4 + 6 = 5 + (4 + 6) = 5 + 10 = 15.