Greatest Common Factor of: 20a^2 and 36a

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you can pull one a out of both terms. a(20a) and a(36). You can then pull a 2 out. 2a(10a) and 2a(18). You can then pull another two out. 4a(5a) and 4a(9). All you're left with then is 5a and 9, which have no common factors. so 4a is the greatest common factor.