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!