399. When the Time is 6 Days, 60 Days, 600 Days or 6000 Days. - Inasmuch as finding the interest when the rate is 6% is the basis for finding the interest for any rate, the 6% case deserves further attention. We have already seen, § 394, that when the time is 60 days the interest at 6 per cent per annum is 1% of the principal, and that the interest is therefore found in this case by pointing off two places in the principal. It is evident that if the time is 6 days (% of 60 days) the interest is found by pointing off three places in the principal; if the time is 600, the interest is found by pointing off one place; and if the time is 6000 days the interest equals the principal. Example 1. Find the interest on $1240 for 43 days, at 6%. Example 2. Find the interest on $9845 for 10 mo. at 6%. Solution. 984 50 interest for 600 days or 20 months Example 3. Find the interest on $2160 for 1 yr. 7 mo. 15 da. at These examples are solved by means of the principles stated formally in § 400. 400. A General Principle. — 1. To find the interest when the time is six days and the rate six per cent, point off three places in the principal. 2. To find the interest when the time is sixty days and the rate six per cent, point off two places in the principal. 3. To find the interest when the time is six hundred days and the rate six per cent, point off one place in the principal. Judicious use of this principle will greatly facilitate finding interest when the rate is 6%. 401. When the Time is an Aliquot Part of 6 Days, 60 Days, or 600 Days. -The method in this case is obvious and is illustrated in the following examples. In the case of 60 days, examples have already been given. See § 396. Example 1. Find the interest on $2480 for 2 days at 6%. 248 interest for 6 days Solution. 83 interest for 2 days (3 of 6 days) Example 2. Find the interest on $3450 for 5 mo. at 6%. Solution. 402. Combinations of Aliquot Parts of 6, 60, and 600 Days. Example 1. Find the interest on $17,650 for 23 days at 6%. interest for 2 mo. Solution. 176 50 Example 2. Find the interest on $125,000 for 11 mo. 17 da. at 6%. Find the interest in each of the following, the rate being 6%. 403. Interest Table. — Bankers and others who need to compute many problems in interest frequently use an interest table, part of which is given below. The interest given in the table is ordinary bankers' interest. The method of using it is illustrated in the following example: Example. Find interest on $38,940 at 5% for 61 days. Interest on $30000 is $254 | 17 In the table no decimal points are given. By a little practice they can be put in without any trouble. The 5% in heavy type at the top of the page indicates that on that page the table gives 5% interest. To find the interest on, say, $700, for 61 days, look under 61 days and in the line in which stands the number 7 in the left margin, where we find the number 05930555. We see at a glance that the decimal point must be placed between the 5 and the 9. That is, the required interest is $5.93. It requires only a little common sense to see that the decimal point could not be placed elsewhere. That is, the interest could be neither $.59, nor $59.31. If the principal were $7000 the interest would be $59.31 and if the principal were $70, the interest would be $.59. In solving a problem by means of such a table, first turn over the pages until you come to the rate of interest of your problem and then under this rate until you come to the number of days of your problem. The solution is then like that on the opposite page. WRITTEN EXERCISES Find the interest at 5% on each of the following: |