# Thread: Very basic question (I've only recently started learning Mathematics beyond multiplication!)

1. Hey all, I'm a little confused on this. I am following a book called 'Foundation Mathematics' and using it in conjunction with Khan Academy.

When you try to find the greatest common factor for 2 (hypothetical) numbers, if for example they have the following prime numbers as factors:

hypothetical number one: 3, 3, 5, 7
hypothetical number two : 3, 5, 11

There are then two primes in common (3 and 5). In the book I am following, it suggests that each of the matching primes are multiplied, and in this case the primes in common are 3 and 5, and to find the GCF you do 3x5= 15 so 15 = HCF, whereas on Khan Academy it suggests the HCF is simply five...

What am I missing??

Thanks!

2.

3. Typo? As far as I know, 15 is the correct answer.

4. Originally Posted by KALSTER
Typo? As far as I know, 15 is the correct answer.
Thanks Kalster, and, should there be three common primes (3,5,11) would the answer then be 165? It just grows as more common primes appear? Sorry for such a simple question but I can't stand not getting the basics down, and I am a new learner.

5. No problem. Your questions are welcome.

Yes, 165 would be the answer. Similarly to find the common factor of three numbers, then it works the same: If 2, 3 and 4 are common between the factors of three numbers, then the answer is 24. If the 2, 3 and 3 are common factors, then the answer is 18.

6. Much appreciated, that has completely cleared things up for me Thank you!

7. Pleasure.

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