The property that would allow you to use mental computation to simplify the problem \( 27 + 15 + 3 + 5 \) is the commutative property of addition. This property states that the order in which you add the numbers does not change their sum.
Using the commutative property, you can rearrange the numbers to group them in a way that makes mental computation easier. For example, you could group \( 27 + 3 \) and \( 15 + 5 \) together, which simplifies to \( 30 + 20 = 50 \).
So the correct answer is: commutative property of addition.