Let's say I contribute a random amount per month to an account. I do this on the 1st of the month. For January through April, my contributions were $100.00, 120.00, 150.00,$ and $100.00$. On April 11th I check the account and find a balance of $500.00$. What is my percentage of interest expressed as APR? (I don't really care about the answer; I want to know how this is calculated.)
Side note - how do I type a dollar sign without messing up the entire page? :)