Interest.
To perform interest on the Comptometer, the following methods are so rapid that the most complex examples in interest can be computed in twenty-five seconds, and an ordinary example in ten seconds.
No one can compute interest one-half as rapid mentally or by using an interest table.
Method of Computing Interest. If the number of days for the whole term is not known, first strike the number of days on the machine, as in addition; then strike the 30 key as many times as there are months, and the 300 key and the 60 key as many times as there are years. The number of days will then be indicated on the register.
Multiply the principal by the number of days, without taking the number of days off of the register, as illustrated on page 9, and divide the product
- By 90, if the rate is 4%;
- By 72, if the rate is 5%;
- By 60, if the rate is 6%;
- By 45, if the rate is 8%;
- By 40, if the rate is 9%;
and point off four places, if there are no cents in the principal; but if there be cents in the principal, point off six places.
Example: Find the interest on $462 for three years, 7 months and 11 days at 8 per cent. First strike 11, as there are 11 days, then strike the 30 seven times, as there are seven months; and strike the 300 three times, and the 60 three times, as there are three years. 1301 is now indicated. on the register. Multiplying the principal by 1301 and 601062 is given as the product; dividing this by 45, and there is a quotient of 1335642⁄45%; point off four places, and $133.56 appears as the answer.
Where the rate per cent is 7, after multiplying the principal by the number of days, multiply this product by striking each 6 key as many times as the corresponding figure of the register indicates. (See rule page 9.) Then divide the amount now indicated on the register by 36, and if there be no cents in the principal, point off five places; but if there be cents in the principal, point off seven places.
Example: What is the interest of $273 for 6 months and 26 days, at 7 per cent? Computing the number of days, 206 is the answer; and multiplying 273 by 206, 56238 appears on the register. As the rate of interest is 7 per cent, multiply by 7 by using the row of 6s as above directed, and 393666 appears on the register. Dividing by 36 causes 109356⁄36 to appear, and pointing off five places, the answer, $10.935, is obtained.
