I know that 1 and 18 are factors of 18, since 1 times 18 is 18. I also know that 2 and 9 are factors of 18, since 2 times 9 is 18. I can also see that 3 and 6 are factors of 18, since 3 times 6 is 18. To find the sum of the factors, I just need to add them up. So the sum is 1 plus 18 plus 2 plus 9 plus 3 plus 6, which is 39. 
What is the sum of the six positive integer factors of 18?