The greatest common factor between two numbers is the biggest number that multiplies into both of them.
Let's factor out 15.
15:1 × 15, 3 × 5.
Let's factor out 64.
64:1 × 64, 2 × 32, 4 x 16, 8 × 8.
Both numbers have 1, so the GCF of 15 and 64 is 1.
Thope this helps you, PM me if you need help understanding. :)