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What is the highest common factor of 21, 36 and 42?
Factors of 21 are 1, 3, 7 and 21.
Factors of 36 are 1, 2, 3, 4, 6, 9, 12, 18 and 36.
Factors of 42 are 1, 2, 3, 6, 7, 14, 21 and 42.
So the highest common factor of these three numbers is 3.
