You would know when one number is also a multiple when the other number is a factor of it as well. For example when we see a prime number like 2, there are no factors for this number so it is just a multiple of 1 and 2.
However for numbers like 15 and 25.
We would know ( if you have your times tables memorized) that 15 is a multiple of 1,3, and 5 and 15. 25 is a multiple of 1, 5, and 25.
