Peter bought his first condo for $89,700.00. A lender will give him the money at 4.12% for 25 years. How much interest will Peter have to pay the lender over the 25 year period and how much will he pay back in total?

There are two basic forms of interest - simple interest and compound interest.  

For simple interest the formula is:

I = PRT (interest equals principal, interest rate and time)

The principal is the actual amount of money you borrow or put into a savings account.