PROBLEM: A bill for $237.43 comes to me marked "20 and 15 off." What is the net bill? WORK AND EXPLANATION: 100%-20% = 80%. 100% 15% = 85%. 85% of 80% = 68%. 68% of $237.43 = $161.45. NOTE. The odd number of cents in the bill might necessitate much writing of figures in making the computation in the ordinary way. By the Wanamaker plan, that is not necessary. PROBLEM: An automobile was listed at $800 with discounts of 20, 10, and 5 off. What is the net price? WORK AND EXPLANATION: On the same plan as in the preceding problem :100% 100% 100% .80.90.95 = .684 .684 of $800 = $547.20. PROBLEM: A merchant receives two bills of $200 each. On one there is a discount of 25%; on the other, 15% and 10%. What must he pay on each, net? INTEREST. TALK: Thus far in percentage, the pupil has not considered time in his calculations. In the study of interest, he will find that it becomes an important factor. He will also be confronted by new expressions and terms. Do not at any time proceed with the work unless the child knows thoroughly the meaning of every term used. It has been proved time and again that where there has been trouble with interest, it was due in most cases to a lack of knowledge of the language used, or of business forms involved. Different conditions call for certain ways of reckoning interest, so our text-books of to-day give a number of methods. Many of them are good but instead of teaching several, it is generally best to adopt and follow only one. Business men generally use what is known as the 60-day method. The 6-per cent method is much used. But rather than teach three or four methods, teach but one. Present the subject of interest this way: No man has everything he wants. He must buy or borrow of others. If he borrows, he pays for the use of the other's property. If a horse is borrowed, the money paid for its use is called hire; if a house is used, rent; and if money, interest. Suppose a man pays 6 a year on each of $100 for the use of the money. The interest will be $6. The principal is $100. Remember: INTEREST is money paid for the use of money. The PRINCIPAL is the money for which the interest is At the end of the year, if the total is paid, the man who lends the money will receive principal plus interest, or $106. This total is called the Amount. Because the man who borrows the money pays 69 on a dollar for a year's use, he is said to pay 6 per cent (6%) of the principal. Remember: The AMOUNT is the total of the principal and the interest. The RATE of interest is the per cent of the principal paid for the use of money for a given time. COMMON INTEREST METHOD. The rate of interest in all the problems is "per annum," or by the year, so the time is reckoned in years and parts of years. Interest at 5% means 5% of the principal for 1 year. If we were to find the interest on a sum of money for 3 yr. 4 mo. 5 da., we would find the interest for yr., then for 1 mo. ( of a year), then for 1 da. (30 of a year). Having the interest for 1 yr. 1 mo. 1 da., it is a simple matter of multiplication to get it for 3 yr. 4 mo. 5 da. 1 PROBLEM: What is the interest on $520 for 1 yr. 3 mo. at 6% ? WORK: When days are included in the time, it is best to find the interest for 1 month, next for 1 day, and then for the number of days, unless the number of days is a reducible fraction of a month. 20 days is a reduci ble fraction of a month, since it equals 38, or of a month. Get the interest for of a month at once, rather than to get it for 1 day and multiply by 20. PROBLEM: What is the interest on $360 for 100 days at 7% ? WORK: When the time can be reduced to a fraction of a year, use cancellation as above. The rule is Find the amount of $400 for 1 yr. 4 mo. at 6%. 3) $24.00 Int. 1 yr. $32.00 Int. 1 yr. 4 mo. $432 Amount. By the 6% method, we find first the interest for $1, then multiply it by the number of dollars in the principal. If 10 is the interest on $1 for a given time, the interest on $20 will be 20 × 10, or $2. At 6% the interest for 1 yr. = .06 of the principal for 1 mo. 1 of .06, or .005 of the principal 1 for 1 da. of .005, or .000 of the principal. Thus, At 6% the interest 30 Also, the interest on $2 for 1 yr. = 2 × $.06 = $.12 on $2 for 1 mo. = 2 × $.005 = $.010 on $2 for 1 da. 2$.0001 = $.000. And the interest on $1 for 2 yr. = 2 × $.06 = $.12. on $1 for 8 mo. 8 x $.005 = $.04. on $1 for 6 da. = 6 $.000 = $.001. Remember: To get the interest at 6%, Multiply $.06, $.005, and $.000 by the number of years, months, and days, respectively. Add the results and multiply by the principal. |
