5
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The highest common factor is 5
The only common factor of 15, 20 and 24 is 1.
The GCF is 5.
-- List all the factors for each number:Factors of 15: 1, 3, 5, and 15.Factors of 20: 1, 2, 4, 5, 10, and 20.-- Look over the lists. Find any numbers that are on both lists:Common factors of 15 and 20: 1 and 5 .-- Pick out the common factor that's biggest:Greatest common factor of 15 and 20 is 5 .
Since 5 is a factor of 20, all the factors of 5 are common to 20.