The greatest prime factor of 75 and 90 is: 5
The prime factors of 90 are: 2,3,5
The prime factors of 90 are 2,3,3,5.
If A is a factor of 90, the GCF is A. If A is a multiple of 90, the GCF is 90. If A is co-prime with 90, the GCF is 1.
The prime factor of 19 is 19. The prime factors of 90 are 2, 3 and 5.
3
Five
The prime factors of 70 are 2, 5, and 7. The prime factors of 90 are 2, 3, 3, and 5. The prime factors that 70 and 90 have in common are 2 and 5. The greatest common factor of 70 and 90 is the product of the prime factors they have in common, so it is 2 x 5 = 10.
170
If 21 is a multiple of the number, the number cannot have 90 as a factor.
That would be 15. Only a number ending in five would be a factor of 75. 45 is not. 35 is not. 25 is not. 15 is, and it also is a factor of 90. This makes it the highest common factor of 75 and 90, although it is not a prime factor.
Respectively, they are 3, 7, 5 and 7 but those four numbers do not have a common prime factor.