(1) Interest is the sum charged for borrowing money for a fixed period of time. Principal is the term used for the money that is borrowed, and the rate of interest is the percent per year of the principal charged for its use. Most of the profits for a bank are derived from the interest that they charge for the use of their own or their depositor’s money.

(2) All problems in interest may be solved by using one general equation that may be stated as follows:

Interest = Principal X Rate X Time

(3) Any one of the four quantities – that is, interest, principal, rate, or time – may be found when the other three are known. The time is expressed in years. The rate is expressed as a decimal fraction. Thus, 6 percent interest means six cents charged for the use of $1 of principal borrowed for one year. Although the time may be less than, equal to, or greater than one year, most applications for loans are for periods of less than one year. For purpose of computing interest for short periods, the commercial year or 360 days is commonly used, but when large sums of money are involved, exact interest is computed on the basis of 365 days.

With what topic is this passage primarily concerned?