Is this how APR works?

Example I owe $310 my APR is %19.99 minimum payment is $10. I just pay the minimum so im at $300. So ill get charged around $5 interest by the time the next billing cycle comes around? If not how much and how do you calculate it. I always pay off my balance but interested in knowing how APR works.Is this how APR works?
This is how to figure your APR interest rate.





take your balance of $310.00 x 19.99% Divide that answer by 365 days and then x's that answer by 30(days in cycle) could be 28 0r 31. here is the problem worked out for you.





310.00 x 19.99% =61.969 divided by 365 = 0.169778082 x30 (days in cycle) =5.0933 so your interest will be $5.09





$310.00


+ 5.09 (interest)


_________


$315.09


- 10.00 (payment)


________


$305.09 This is your new balance after you make your minimum payment of $10.00





Now for next month do the same thing but use $305.09.