Question

The greatest common factor of 15 and 45 is?

Answer 1

try using 5. because 5(3) is 15 and 5(9) is45

Answer 2

look at prime factorisation:
15=3*5
45=3^2*5

so mutual primes are 3 and 5
and GCF is 5*3 = 15
I'm 100% sure

and try the Maths section next time :)