Do you mean the common factor? In order to have a common denominator you need fractions
The common factor number that goes into all of those numbers evenly without a remainder.
You might also mean common multiple which is the number that all of the number can be multiplied evenly by.
Common factors:
Factors of 7
7 = 1 times 7
7 is a prime number which means it only has factors of itself and 1
factors of 5
5 = 1 times 5
5 is also a prime number
Factors of 4
4 = 4 times 1 but,
4 = 2 times 2
factors of 4 are:
4,2 and 1
Factors of 2
2 = 2 times 1
2 is a prime number. Its only factors are itself and 1
the only common factor of 7,5,4 and 2 is:
1
It is the only number on all of the lists.
Hope this helps. If that isn't what you needed you can ask another question.