The answer is the Associative Property of addition.
Associative property of real numbers is related to how we use the parenthesis in the expression while adding or multiplying, we always get the same answer.
like a + b + c = (a + b) + c = a + (b + c)
The given equation is 4.5 + 9.8 +1.5 + 3.2 = (4.5 + 9.8) + (1.5 + 3.2) = (4.5 + 1.5) + (9.8 + 3.2)