What is the sum of all the common factors of 10 and 45 ?
There is probably a formula for this, but let's do it by hand because they're easy numbers.
Factors of 10: 1, 2, 5, 10,
Factors of 45: 1, 3, 5, 9, 15, 45,
Then we look at the factors they share and we get: 1, 5,