The greatest common factor of 12, 78, and 90 is 6.
Factors are the numbers, that when multiplied with another number, equal the product.
Factors of 12: 1, 2, 3, 4, 6, and 12.
Factors of 78: 1, 2, 3, 6, 13, 26, 39, and 78.
Factors of 90: 1, 2, 3, 5, 6, 9, 10, 15, 18, 20, 45, and 90.
The Greatest Common Factor is the highest factor shared among the products. Thus proving 6 is the GCF.
I hope this helps!