INTEREST DUE

Good day! I just want to seek for help regarding the proper computation of interest on auto loan. I am working in a company and my boss is currently having a loan under her name but her aunt is the one who is using the vehicle. Her aunt issued her PDCs to cover the 5 years term. Her aunt was not able to pay/fund the Dec amortization up until now (internal arrangement only). Paid na yung sa bank side, ang boss ko yung nag advance payment. PDCs due on Jan-May 2018 were all good. How do we go about the computation of interest po na babayran ng aunt ng boss ko sa kanya? Please help. Thank you.