Averaging The Interest Rate

If you wish to know what your annual rate of interest is on a combination of mortgages, that is the tool to do it.

Mortgage amount - The amount originally borrowed from a lender (PRINCIPAL).
Annual interest rate - The amount charged by a lender to a borrower as 'rent' for the use of the lender's money. Interest charges generally accrued as a percentage of the amount borrowed. The interest rate is usually quoted in percent per year.