Overview
Percents are based on the decimal system and have many useful applications in business, sales, and finance. Other applications of percents include percent increase and decrease.
Estimating Percentages
Many of the most useful applications of percents involve estimating and calculating percentages for sales tax and tips at a restaurant. Suppose the sales tax is 8.7% in a particular town. That means that for any taxable item, the cost to buy it is the cost C plus .087C, calculated automatically at the cash register. It is always better to estimate high to make sure there is enough money for a purchase, and it is easier to use a value such as 10%. Then, the price of that item is the cost C plus .10C. If the cost of a CD is $15.99, estimate $16.00, and add 10% of $16.00 or $1.60, so the estimated cost will be a little less than $16.00 + $1.60 or $17.60. If the cost is higher, the estimated tax will also be higher. Suppose a DVD is $24.00. Its estimated price with tax will be $24.00 + $2.40 or about $26.40.
Tips
Estimating tips works on the same principle, except the percentage of tip to leave is up to the diner, and depends on the quality of service and the type of service. Suppose the waitperson brings the check and it is $25.36, including tax. The quality of service is good, and the diners decide to leave a 20% tip. The cost of the meal will then be the price P plus 0.20C. It is easier to estimate the tip by adding 0.10C + 0.10C, or $2.54 + 2.54, so the cost of the meal is $25.36 + $5.08 or $30.44.
Commissions and Fees
Commissions are typically a percentage of total sales and are either added to a base salary or paid as commission only. Suppose that Lori works in a salon, and her employer pays her $9.50 an hour plus a 1.5% commission on all products she sells. Her gross wages after a 40-hour work week are $9.50 times 40, or $380.00. Suppose she sells $125.00 in products during that week. Her commission will be $125.00 times 0.015, or $1.88. (When using percents in a calculation, change the percent to a decimal by multiplying by 1/100 and using decimal points.) $380.00 +$1.88 = $381.88.
Calculating Simple Interest
Simple interest follows a formula of I=Prt, where I is the amount of interest, P is the principal amount of money invested or loaned, r is the interest rate, given as a percentage, and t is the amount of time in years. It is only paid on the principal. In order to find the amount of simple interest paid on a loan of $1000.00 at a rate of 15% over 2 years, first substitute the known values in the formula such that I = $1000.00 (0.15)(2). Again, the 15% is changed to a decimal 0.15. Solving the equation, I = $300.00.
Percent Increase and Decrease
Percent change refers to the amount of increase or decrease given as a percentage. Percent increase is often referred to as a markup, and percent decrease is a discount. Suppose the regular price of an item is $12.00 and it is being sold at 30% off. In this case, the sale price is equal to the regular price P minus the discount of 30% P, or the sale price is equal to $12.00 – (0.30) P or $12.00 – 3.60 = $8.40.
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