Answer: 5
Step-by-step explanation: To find the (GCF) or greatest common factor of 15 and 20, begin by finding all of the factors of 3.
Since 15 ÷ 1 is 15, this means that 1 and 15 are factors.
Since 15 ÷ 3 gives us 5, this means that 3 and 5 are factors.
Also, 15 ÷ 5 and 15 ÷ 15 would be factors, but we don't list repeat factors.
Next, we find all of the factors of 20.
Since 20 ÷ 1 is 20, we know that 20 and 1 are factors.
Since 20 ÷ 2 is 10, we know that 10 and 2 are factors.
Since 20 ÷ 4 is 5, we know that 5 and 4 are factors.
Now we look for the largest factor that is shared by the two lists which is 5.
Therefore, the greatest common factor of 15 and 20 is 5.
