The largest number, which is the factor of two or more number is called the Greatest Common Factor (GCF). It is the largest number (factor) that divide them resulting in a Natural number.
GCF of 15 and 25
Let us first find the factors of 15 and 25
Factors of 15: 5 and 3
Factors of 25: 5
Hence the GCF among 15 and 25 is 5