Mark M. is correct that only simple interest is involved.
However, I would interpret 7.8% to be the annual interest rate.
The monthly (nominal) interest rate is 7.8/12 (=.0065) and the true monthly interest rate is
1.0781/12-1. Please note that I am aware that almost everybody teaches nominal interest rate.
Further note:
(1.0065)12=1.08085, not 1.078.