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5-81.

Even numbers are closed under addition, because the sum of two even numbers is also even.

For example,
4 + 4 = 8
10 + 10 = 20
36 + 36 = 72

What happens if you mulitply two even numbers together?

For example,
(4)(4) = 16
(10)(10) = 100
(36)(36) = 1296