# Please open your laptops, log in to the MyMathLab course web site, and open Quiz 2.5. IMPORTANT NOTE: If you have time left after you finish the problems.

## Presentation on theme: "Please open your laptops, log in to the MyMathLab course web site, and open Quiz 2.5. IMPORTANT NOTE: If you have time left after you finish the problems."— Presentation transcript:

Sections 2.6 and 2.7 More Problem Solving

Solving Percent Problems A percent problem has three different parts: 1. When we do not know the amount: n = 10% · 500 Any one of the three quantities may be unknown. amount = percent x base 2. When we do not know the base: 50 = 10% · n 3. When we do not know the percent: 50 = n · 500 Note: Per cent means per 100. For example, 17% = 17/100 = 0.17.

Solving a Percent Problem: Amount Unknown What is 9% of 65? 5.85 is 9% of 65 Example:

Solving a Percent Problem: Base Unknown 36 is 6% of what? 36 is 6% of 600 Example:

Solving a Percent Problem: Percent Unknown 24 is what percent of 144? Example:

Percent Problem Tip: If youre having trouble doing percent problems that give you a new value after a certain percent increase or decrease from an old value (such as sales tax problems), try thinking about it this way: Think about when you go shopping to buy, say, a TV. Usually you know how much the TV costs, for example \$400, and the percent tax rate, for example 5.5%. Normally what you do (or the salesclerks computer does) is calculate the TOTAL COST by taking 5.5% of \$400, then adding that amount back onto the \$400 price of the TV to get the total cost to you.

The working equation is PRICE + TAX = TOTAL COST. In words, heres what you did (after writing the 5.5% as a decimal, 0.055): PRICE +.055 times PRICE = TOTAL COST Plugging in the numbers, we get 400 +.055 x 400 = 400 + 22 = 422. Notice that youve multiplied the OLD VALUE (the price before tax) by the.055.

The same basic format applies to anything with a percent increase or decrease from an original amount: Old amount +/- % of old amount = new amount (Remember to write the percent as a decimal.) This equation works for raises in pay, population increases or decreases, and many other percent change problems, especially where youre given the new amount and the percent change and you need to work backwards to find out the old amount.

Example: After a 6% pay raise, Noras 2013 salary is \$39,703. What was her salary in 2012? (Round to the nearest dollar). Solution: Recall the equation: Old amount + % of old amount = new amount The old amount is her 2012 salary, which is unknown, so well call it X. This gives us the equation X + 0.06X = 39703

Example (cont.) After a 6% pay raise, Noras 2013 salary is \$39,703. What was her salary in 2012? (Round to the nearest dollar). 1X + 0.06X = 39703 This simplifies to 1.06X = 39703 Divide both sides X = 39703 by 1.06 to get X. 1.06 Answer: Her 2012 salary was \$37,456

Now check your answer: 37456 +.06 x 37456 = 37456 + 2247 = 39703 NOTE that this DOES NOT give you the same answer as if you subtracted 6% of the new salary (39703) from the new salary. Try it and youll see that it doesnt work. (Its not real far off, but enough to give you the wrong answer, and the bigger the percentage, the farther off youll be.) On these kinds of problems, we wont give partial credit for those close answers on tests.

Sample problem from todays homework:

Solving Mixture Problems The owner of a candy store is mixing candy worth \$6 per pound with candy worth \$8 per pound. She wants to obtain 144 pounds of candy worth \$7.50 per pound. How much of each type of candy should she use in the mixture? Solution: Let n = the number of pounds of candy costing \$6 per pound. Let 144 – n = candy costing \$8 per pound. Example:

Use a table to summarize the information. Number of Pounds Price per Pound Value of Candy \$6 candy n66n6n \$8 candy 144 n 8 8(144 n) \$7.50 candy 1447.50144(7.50) 6n + 8(144 n) = 144(7.5) # of pounds of \$6 candy # of pounds of \$8 candy # of pounds of \$7.50 candy

6n + 8(144 n) = 144(7.5) 6n + 1152 8n = 1080 1152 2n = 1080 2n = 72 n = 36 She should use 36 pounds of the \$6 per pound candy. She should use 108 pounds of the \$8 per pound candy. (144 n) = 144 36 = 108

Check: Will using 36 pounds of the \$6 per pound candy and 108 pounds of the \$8 per pound candy yield 144 pounds of candy costing \$7.50 per pound? 6(36) + 8(108) = 144(7.5) ? 216 + 864 = 1080 ? 1080 = 1080 ?

Distance Problems: distance = rate · time or d = r · t When the amount in the formula is distance, we refer to the formula as the distance formula.

The assignment on this material (HW 2.6/7) is due at the start of the next class session, and there will be a short quiz on it as usual, either at the start or end of the class session.. Lab hours: Mondays through Thursdays 8:00 a.m. to 6:30 p.m.