20 and 30
All numbers have at least one common factor.
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All two digit prime numbers have two factors only, starting with 11, 13, 17, 19.......
Almost all of the two digit composite numbers have two one digit factors that have a sum.
60, 72, 84, 90 and 96 all have 12 factors; tied for the most of two-digit numbers.
The number one has one factor. Prime numbers have two.
Both 10 and 20 are two-digit numbers that are factors of 20.
1 is the number less than fifty with the fewest number of factors. All other numbers have at least two factors (1, and the integer itself).
Four. They are 14, 21, 28, and 42.
All numbers that have exactly two factors are prime numbers whereas composite numbers have more than two factors.