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