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hundredths. One percent is one one-hundredth. The symbol for percent is %. Thus, 19% of 3 m is 0.57 m, or 57 cm. When speaking of thousandths of a whole—that is, tenths of one percent—the terms “per mill” or “parts per thousand” are sometimes used; in Russian usage the symbol ‰ is used in this case. Parts of quantities are commonly expressed in percent in economic and statistical calculations and in many areas of science.
One important use of the concept of percent is the expression of interest rates. For simple interest, the simple interest formula is used: if the interest on an amount a is p% per year (or some other interval of time), then after t years the total of a and interest is x = a(1 + pt/100). It is assumed here that at the end of each year the interest income for the year is withdrawn; the interest in the following year is thus calculated from the initial amount. If, however, the interest is added to the initial amount and the interest for the following year is consequently calculated from the increased amount, we speak of compound interest. In this case, the total of a and interest after t years is calculated by the formula x = a(1 + p/100)t. When interest is calculated for a portion of a year, the year is usually taken as 360 days, and each month as 30 days; interest calculated in accordance with this convention is known as ordinary interest.
Compound interest and related concepts are used in many areas of economic activity and bookkeeping—for example in commercial and savings banks. They are also used in various statistical computations; in particular, they are used in determining the average annual rate of relative increase or decrease over extended periods of time, such as periods of five or ten years.