Answer:
Only one student, Melinda used it right.
Step-by-step explanation:
Associative property lets us place our brackets the way we want and the result would be unchanged.
Here: (13 + 15 + 20) + (20 + 47 + 18).
So, we can place the brackets in any form, like (13 + 15) + (20 + 20) + (47 + 18).
There are many ways to do it. Melinda's was is yet another example.
Note that Associative property does not hold for subtraction.
