They have the prime factors 2 and 5 in common.
19 and 90 have no common prime factors.
81 and 200 have no common prime factors.
The prime factors of 80 are 2 and 5. For them to be common, they need to be compared to another set of prime factors.
The prime factors of 36 are 2 and 3. To be common, they need to be compared to another set of prime factors.
The prime factors of 18 are 2, 3, and 3. The prime factors of 90 are 2, 3, 3, and 5. The prime factors they have in common are 2, 3, and 3. Since 18 is a factor of 90, its prime factors will be the prime factors that the two numbers have in common.
The common prime factors are 2 and 3
2 and 3 are the only common prime factors
The prime factors of 110 are 2, 5 and 11. For them to be common, they need to be compared to another set of prime factors.
When two or more numbers have one or more of the same primes as factors, those prime factors are called common prime factors. For example, the prime factors of 6 are 2 and 3, and the prime factors of 21 are 3 and 7, so 3 is a common prime factor of 6 and 21.
There are infinitely many prime factors.
All numbers have factors. Some factors are prime numbers. These are known as prime factors. Some numbers have some of the same prime factors as other numbers. These are known as common prime factors. 3 is a common prime factor of 12 and 15.