1268% APR
First, let's see what interest is charged over a 1 week period.
(5.10 - 4.10)/ 4.10 = 1.00/4.10 = 0.2439 = 24.39%
So we have 24.39% for 1 week. But APR stands for Annual Percentage Rate. And since there's 52 weeks in a year, we need to multiply that weekly percentage rate by 52. So:
24.39% * 52 = 1268.28%
Ouch, usury at it's greatest. You can get loans from Friendly's at the low low rate of only 1268% APR. I'd suggest going elsewhere for your financial needs.