Interest is a fee charged for the use of money. For example, if you place money
in a savings account or the bank, or some other organization will pay you money
to use your money for its purposes. On the other hand, if you borrow money
or use a credit card to purchase an item, you must pay the bank, store, or other
lending institution a fee (interest) for the use of its money.

Calculate Simple Interest:
Time is expressed in the same units as the rate; if r is an annual rate, then t is in years.
Simple interest (i) = principal (p) x rate in percent (r) x time (t).
Maturity value = principal + interest
p is principal of the loan (or borrowed) amount or invested amount (face value);
i is the amount of interest paid for the loan or earned on the investment; r is the percent rate of interest paid for the use of someone else's money or earned for lending the money