# 1 Percentage: A commonly used relative quantity..

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1 Percentage: A commonly used relative quantity.

How large is one quantity relative to another quantity? 2

How large is the compared quantity [cq] relative to the reference quantity [rq]? 3

4 Percentages simplify comparison of two or more groups of different sizes. Remove the effects of the different sizes by placing them on the same standard: “of one hundred”

5 Compared quantity ═ Percentage Reference quantity 100 Or 100 * cq/rq = percentage Or 100 * proportion = percentage General model

Steps in solving percentage problems: A general model In practice, steps often are skipped or done out of sequence in familiar or easy problems. The model is useful for showing the basic logic of solving even easy or familiar problems. The model is useful for coping with complex or confusing problems. 6

7 Steps in solving percentage problems: 1. Figure out the type of problem

8 Steps in solving percentage problems: 1.Figure out the type of problem. 2.Figure out which numbers go where.

9 Steps in solving percentage problems: 1.Figure out the type of problem. 2.Figure out which numbers go where. Do necessary preliminary computations. 3.Solve using elementary algebra

10 Steps in solving percentage problems: 1.Figure out the type of problem. 2.Figure out which numbers go where. Do necessary preliminary computations. 3.Solve using elementary algebra a.An equation is like a scale: whatever you do to one side you must do to the other.

11 Steps in solving percentage problems: 1.Figure out the type of problem. 2.Figure out which numbers go where. 3.Solve using elementary algebra a.Equation is like a scale: whatever you do to one side you must do to the other. b.Isolate the unknown on one side of the equation.

12 Steps in solving percentage problems: 1.Figure out the type of problem. 2.Figure out which numbers go where. 3.Solve using elementary algebra. 4. Do the arithmetic.

13 Types of problems: 1. Part-whole problems 2. “Percentage of” problems 3. Percentage difference problems 4. Percentage change problems

14 Part-whole problems The whole is the reference quantity; the part is the compared quantity. For part-whole problems only, the percentage can never be more than 100%.

15 1.a: Part-whole problems where the percentage is unknown (x)

16 7 of 28 students in LSP 120 are juniors or seniors. What percentage of the class is juniors or seniors?

17 1.a: Part-whole problems where the percentage is unknown (x) cq/rq = x/100

18 1.a: Part/whole problems where the percentage is unknown (x) cq/rq = x/100 100 * (cq/rq) = x

19 1.b: Part-whole problems where the part is unknown (x)

20 14% of 28 LSP 120 students are in colleges other than LA&S. How many non-LA&S students are in the class?

21 1.b: Part-whole problems where the part is unknown (x) x/rq = p/100

22 1.b: Part-whole problems where the part is unknown (x) x/rq = p/100 x = p/100 * rq

23 1.c.: Part-whole problems where the whole is unknown (x).

24 16 students in an LSP 120 class are women. They make up 57% of the class. How many students are in the class?

25 1.c.: Part-whole problems where the whole is unknown (x). cq/x = p/100

26 1.c.: Part/whole problems where the whole is unknown (x) cq/x = p/100 cq=(p/100) * x

27 1.c.: Part/whole problems where the whole is unknown (x). cq/x = p/100 cq = (p/100) * x cq/(p/100) = x

28 A September, 2005, Gallup Poll found that 58% of a sample of 818 Americans disapproved of President Bush’s performance. How many people in the poll gave Bush a disapproving rating?

29 Of a sample of Chicagoans, 780, or 25%, are smokers. How many people were in the sample?

30 “Percentage Of” problems

31 “Percentage Of” problems These problems concern the direct comparison of two numbers where neither is a subset (part) of the other. The number of Sox wins compared to number of Cubs wins. The price of gas now compared to the price last year.

32 “Percentage Of” problems Characteristic phrasing: “X is what percent of Y?” “What percent of Y is X?” “In percentage terms, how large, small, (etc.) is X relative to Y?” Does not include problems with “greater than/less than” language

33 “Percentage Of” problems 100 * (x / y) = percentage

34 Percentage difference problems

35 Percentage difference problems Concern the difference between two numbers [n1, n2] relative to one of the numbers [n2]. (The numbers represent the values of two cases on the same variable at the same time.)

36 Percentage difference problems The difference [N1-N2] is the compared quantity [cq] The “than” number [N2] is the reference quantity [rq].

Percentage difference problems The standard verbal cues to a percentage difference problem are (1) the word percent or percentage and (2) words indicating relative size, such as “more than” or “less than” or “greater than” or other synonymous phrases. 37

Percentage difference problems (n1-n2)/rq = p/100 100*(n1-n2)/rq=p 38

Percentage difference problems In word problems, the amount of the difference [cq] often is not presented. So it must be computed from the information given in the problem. 39

40 Percentage difference problems Afghanistan has an area of 647,500 sq km. The area of Illinois is 150,007 sq. km. By what percentage is Afghanistan larger than Illinois?

41 “percentage of” problems and percentage difference problems: Rules of inter-conversion Convert percentage difference to “percentage of” by adding to 100. ( Remember: If cq is less than rq, the percentage is negative, so addition is subtraction) Convert “percentage of” to percentage difference by subtracting 100

42 Examples of converting between “Of” problems and “more than” problems “25% more than” is the same as “125% of” “17% less than” is the same as “83% of” “250% of” is the same as “150% more than” “60% of” is the same as “40% less than”

43 Percentage change problems

44 Percentage change problems These are like percentage difference problems but involve comparing the same thing at two different times [“old” and “new”]. “New” is n1 and “old” is n2. Compute the compared quantity by subtracting the value for the later time [old] from that for the earlier [new]. The number for the earlier time is the reference quantity.

Percentage change problems (new-old)/old = p/100 45

46 Percentage change problems In 2000, the world population was 6.1 billion. In 1990, it was 5.3 billion. What was the percentage rate of change in the world’s population between 1990 and 2000 than in 1990?

47 Comparisons of types of problems Percentage can exceed 100% for “percentage of” and percentage difference and “percentage change” problems but never for “part-whole” problems

48 Comparisons of types of problems “part-whole” and “percentage of” problems normally require no calculations prior to plugging in numbers; percentage difference and percentage change problems do

49 Special hint For percentage difference and percentage change problems: If the problem specifies the percentage and the compared quantity, but reference quantity is unknown Solve by converting 3to “percentage of” problem

50 Example: Professor Muddle lost 15% of his weight on his new diet. He now weighs 180 pounds. How much did he weigh originally? To solve, you must restate the problem: His current weight (180 pounds) is 85 percent of his pre-diet weight.