To find the greatest common factor (GCF) of 15, 20, 40, and 90, we can start by finding the prime factorization of each number:
15 = 3 x 5
20 = 2 x 2 x 5
40 = 2 x 2 x 2 x 5
90 = 2 x 3 x 3 x 5
Next, we can find the common factors of these numbers by looking at the prime factors that they share.
The prime factors that 15, 20, 40, and 90 have in common are 2, 3, and 5.
To find the GCF, we can multiply these common factors together:
GCF = 2 x 3 x 5
GCF = 30
Therefore, the greatest common factor of 15, 20, 40, and 90 is 30.