Every number has at least two factors. The two factors are one and the number itself. Therefore, 1 and 20 are factors of 20. To find other factors of 20, note that it is an even number because it ends in a 0. Since it is even, to find another factor, divide by 2. 20/2=10, so 2 and 10 are factors of 20.
According to the divisibility rules, because the sum of the digits in the number 20 is not divisible by 3, neither is the number 20. But 10 is an even composite number, so it can be divided by 2. 10/2=5, and 20/5=4, so 4 and 5 are also factors of 20. The set of factors for 20 is {1, 2, 4, 5, 10, 20}.
As with 20, 40 has the factors 1 and 40. Because it is even, it is divisible by 2. 40/2=20, so 2 and 20 are factors. Using divisibility rules, 40 is divisible by 5 and 10 because it ends in a 0. 5*8=40 and 10*4 = 40, so 4, 5, 8 and 10 are factors of 40. Therefore, the factors of 40 are {1, 2, 4, 5, 8, 10, 20, 40}.
The common factors of 20 and 40 are {1, 2, 4, 5, 10, 20}. These numbers will evenly divide both 20 and 40.
The GCF is the greatest (largest) common factor between two numbers. For 20 and 40, this number is 20. Finding the GCF is helpful in reducing fractions.