1, 2, 3 and 6
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It is common to group factors in pairs with parentheses surrounding each pair.
The common prime factors of 18 and 30 are 2 and 3.
Since 1, known as the "multiplicative identity", cannot be counted as a factor, there are many pairs of numbers that do not have common factors, such as 2 and 3, 2 and 5, and 3 and 5. There are many pairs of composite (not prime) numbers without common factors, such as 4 and 9. Also, since prime numbers do not have factors, any pair of primes cannot have a common factor, though a pair in which one number is prime may, e.g. 5 and 10.
A single number does not have a common factor. Common factors are factors that two or more numbers have in common. The greatest common factor of a pair of numbers over 50 could be any number, depending on the pair of numbers. The greatest common factor of 51 and 100 is 1. The greatest common factor of 51, 52, 53, 54, 55, and 56 is 1. The greatest common factor of 52 and 100 is 2. The greatest common factor of 57 and 102 is 3.