"Common" means "same for all". There's never anything 'common'
about a single thing, and no 'common' factors of a single number.
There are no COMMON factors]between '9' & '25'. '9' has the factors os 3, & 9. '25 has the factors of 5, & 25/
The COMMON factors of 9 depend upon the OTHER numbers with which 9 has factors in COMMON. However, they will be one or more of the factors of 9 which are 1, 3, 9.
Since 9 is a factor of 36, all of its factors are common.
Since 9 is a factor of 27, all of its factors are common.
Since 9 is a factor of 81, all of its factors are common.
All multiples of 9 share common factors with 9.
Since 9 is a factor of 18, all of its factors are common.
Since 9 is a factor of 27, all of its factors are common.
1, 3 and 9.
Since 9 is a factor of 63, all of its factors are common.
The common factors of 6, 21, and 39 are 1 and 3
1, 3 and 9.