What is the greatest common factor of 15 and 40?

The GCF of 40,15 is 5

15 = 3 x 5

40 = 2 x 2 x 2 x 5

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Another way to see it, which may be easier for you to understand is:

15 = 5 x 3 (5 is a factor and 3 is a factor)

40 = 5 x 8 (5 is a factor and 8 is a factor)

So the answer is 5, which is the largest number (greatest common factor) that will divide evenly into 40 and 15.
The GCF is 5.