How do you find the GCF of 9 and 15 and 24?

1 Answer


GCF = 3


A factor is a number that when multiplied by another number, gives us the number in question. So for instance 3 is a factor of 9 because #3xx3=9#.

The GCF is the highest number that is a factor of all three of our "test subject numbers".

So let's do a prime factorization of all three to start things out and I'll colour the different prime values in different colours:


Ok, now let's find what is common.


There are 2s in 24 but not in 9 or 15, so there are no 2s in our GCF.


There is one 3 in 24 and in 15 and two in 9 - so one 3 is common to all three numbers.


There is a 5 in 15 but not in 9 or 24, so the GCF doesn't have a 5.

And there are no more prime numbers.

This means our GCF = 3.