Need help with this question. I know the answer but I have difficulty understanding how to get it.

"Suppose that you take out an unsubsidized Stafford loan on Sept 1 before your senior year for $5500 and plan to begin paying it back on Dec 1 after graduation (so you will have had the loan 15 months, including the six month grace period after leaving school). The interest rate is 6.8%. How much will you owe then, and how much of that will be in interet?"

Answers are5967.50,467.50

I'm pretty sure the formula is I=Prt, but again when I solve for 'I', I dont get 5967.50!