고등학교 영어Ⅰ (YBM 박준언) 연계 글

For the past two years, Yeonju, a high school junior, has been depositing part of her monthly allowance into a savings account. Her deposits are modest, rarely above 20,000 won, and she is often tempted to withdraw the money for snacks. When her annual statement arrived last month, she found herself staring at a single figure: a full year's interest was worth less than a movie ticket. Such a meager reward made her wonder whether saving was worth the effort.

The answer depends on the difference between two methods of calculating interest. Under simple interest, a bank pays interest only on the principal, the sum that was originally deposited. Under compound interest, each interest payment is added to the principal, so that the following payment is calculated on a steadily growing sum. Suppose that 200,000 won earns 3 percent a year: after one year the account holds 206,000 won, so the second year's interest is 6,180 won instead of 6,000. The extra 180 won is interest on interest, and from then on the interest itself keeps generating further interest. Although such gains appear trivial at first, they accumulate, and over several decades they can exceed the original deposit. Without her noticing, Yeonju's own savings have been earning interest on interest all along.

Yeonju also came across a handy shortcut, the rule of 72: dividing 72 by the yearly interest rate gives the approximate number of years money needs to double. At 8 percent, for example, money doubles in roughly nine years. The rule is centuries old; in 1494, the Italian mathematician Luca Pacioli published a book that summed up the mathematics of his day and included the rule. Pacioli presented it as a general method for estimating how long a loan would take to double. Since the rule yields only an approximate answer, it serves as a quick check rather than a precise formula.

Yeonju, who is seventeen, then tested the rule herself on her savings, which now total 500,000 won. At an imaginary rate of 4 percent a year, the money would double about every 18 years, reaching about one million won by age 35 and two million by 53. If she postponed saving the same amount until 35, she would have only half as much at 53.

Since then, Yeonju has been thinking about her allowance differently. Actual interest rates rise and fall, so she knows the rule offers an estimate, not a promise. Still, whenever she is tempted to empty her account, she reminds herself that untouched money has more time to double.