something i learnt this week in math was how to find the GCF, (greatest common factor) and its basically where you have two different numbers and with both of them you list the prime factors of each number. Then if you find multiple numbers they have in common you multiply them to get the GCF but if you just find one common factor then that number is your common factor.

example:

35                   42

7  5                 2 21

3  7                                gcf=7