To complete the expression, let's go through the steps. The original expression is:

$5 + 6q + 5$

Step 1: Combine like terms

The constants $5$ and $5$ can be combined.

$5 + 5 = 10$

So, the expression simplifies to:

$10 + 6q$

Step 2: Missing property

The operation used here involves grouping and combining like terms, which is an application of the Associative property of addition. This property allows you to regroup and add numbers in any order, focusing on the fact that $5 + 5$ can be done before anything else.

Thus, the completed expression is:

$10 + 6q$

The property used is Associative property of addition.

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Would you like further details or explanations on this? Here are some related questions:

1. What is the Associative Property of Addition?
2. Can the Commutative Property apply to both numbers and variables?
3. What is the Distributive Property and how does it differ from others?
4. How do you combine like terms in algebraic expressions?
5. What role does the Associative Property play in solving equations?

Tip: When combining like terms, always group constants and variables separately to simplify expressions efficiently.