Percents - GMAT Math Study Guide
Table of Contents
The Concept of Percentages
Percent, when broken apart, literally means per 100. A percent represents a part of 100. For example, 20% means 20 per 100.
Since a percent is an amount per 100, percents can be represented as fractions with a denominator of 100.
55% = 55/100
100% = 100/100
125% = 125/100
250% = 250/100
0.5% = 5/1000
When a percentage is represented as a fraction, it can be added, subtracted, multiplied, and divided just like any other fraction.
A percent can be represented as a decimal. The following relationship characterizes how percents and decimals interact.
Stated in sentence form: To move from percent form to decimal form, move the decimal point two slots to the left.
Consider the following examples:
5 * (.01) = .05
Note: .05 is the result of taking 5 and moving the decimal point two slots to the left.
What is 130% represented as a decimal?
130 * (.01) = 1.3
Note: 1.3 is the result of taking 130 and moving the decimal point two slots to the left.
What is 0.5% represented as a decimal?
0.5 * (.01) = 0.005
Note: 0.005 is the result of taking 0.5 and moving the decimal point two slots to the left.
When a percentage is represented as a decimal, it can be added, subtracted, multiplied, and divided just like any other number.
The following chart lays out the relationship between percents, fractions, and decimals.
Percent Change vs. Percent Of
While most students find percentages to be an easier topic than one such as combinatorics, some individuals initially trip on the difference between a percent change and a percent of a number. Practically, this is the difference between saying "the price jumped 50%" and "the current price is 150% of the old price." Both of these phrases refer to the same amount, but are stated differently.
Percents are commonly used to measure or report the change in an amount. For example, a news reporter might say, "stocks rose 1.5% today" or a demographer might write, "minority representation in the population fell 3.5% during the past decade." The formula for calculating percent changes is:
This formula can also be expressed in decimal form. In other words, the following formula calculates the percent change between two numbers and represents this change in decimal form.
The following examples illustrate the use of this formula.
End Value = 9
Start Value = 10
Percent Change [as a percent] = ((9 - 10)/10) * 100 = -.1 * 100 = -10%
End Value = 60
Start Value = 50
Percent Change [as a percent] = ((60 - 50)/50) * 100 = .2 * 100 = 20%
It is possible to calculate the percent change of a percent. Consider the following example:
End Value = 85% = .85
Start Value = 75% = .75
Using Percents: Percent Change [as a percent] = ((85% - 75%)/75%) * 100 = 13.3% * 100 = 13.3%
Using Decimals: Percent Change [as a decimal] = ((.85 - .75)/.75) * 100 = .133
A Common Mistake in Working With Percent Decreases
Some students confuse a percent decrease of a certain percentage with finding the percent of a certain amount. The following example elucidates this confusion:
Common Mistake: IndexToday = 5000(.45)
This calculation yields 45% of last year's index value. However, the question pertains to a 45% fall. Since the index's value fell 45%, its current value is 100% - 45% = 55% of last year's index value.
Correct Calculation: IndexToday = 5000(1-.45) = 5000(.55) = 2750
Another common use of percents is as a measure of another number. For example, a stock analyst might say, "MicroMake's stock is trading at 130% of MacroMake's stock price." Similarly, a political historian might say, "President George W. Bush's approval rating in late November 2004 was about 50%, which is about 55% of his approval rating in late September 2001." In these instances, percents are being used not to describe change, but to compare amounts or quantities.
When working with percents that are used to compare different quantities, it is often best to translate each percent into decimals and set up equations or ratios. Consider the following examples:
Translate 50% into decimal format: 50% = .5
Translate the question into an equation: .5(40) = ?
.5(40) = 20
The following is a slightly more difficult example:
Let X = the percent as a decimal
Translate the question into an equation: X(80) = 20
X = 20/80 = 1/4 = .25
Translate X into a percent: .25(100) = 25%
Percents can also be used to compare the size of percents. Consider the example with President George W. Bush's approval rating mentioned above.
Let A = President Bush's approval rating in late September 2001
Condense Question Down to Simplify: 50% is 55% of A
Translate Into Equation: .50 = .55A
A [as a decimal] = .9
A [as a percent] = .9(100) = 90%
If a number rises by 30% and then falls by 35%, by what percent did it change from beginning to end? The topic of recursive (or successive) percents addresses this question. Consider an example:
Let DowBeginning of 2004 = X
DowEnd of 2007 = X(1 + 30%) = X(1.3)
DowEnd of 2008 = [X(1.3)](1-.35) = X(.845)
Percent Change = (End - Start/Start)*100
Percent Change = (X(.845) - X/X)*100 = -15.5%
Strategy: Picking Numbers (Especially 100)
Many students find it easier to solve problems involving percents by picking numbers instead of using theoretical variables. The previous question can be solved this way:
Let DowBeginning of 2004 = 100 [pick the number 100 instead of using a variable]
DowEnd of 2007 = 100(1.3)
DowEnd of 2008 = 100(1.3)(1-.35) = 84.5
The choice of 100 as a value for the Dow at the beginning of 2004 makes calculating the percent change from 2004 through 2008 much easier, as the next step should indicate.
Percent Change = (End - Start/Start)*100
Percent Change = (84.5 - 100/100)*100 = -15.5%
Interest Rate Problems
One rather common and important application of percents is the topic of interest rates and money. An important formula that relates interest, principal, and time follows:
I = PRT
I = Interest Payment
P = Principal
R = Interest Rate
T = Time Period
T = 1 since the question asks for the interest, I, in the first year (i.e., a one year time period--not the entire 10 year time period)
P = $100,000
R = 5% = 0.05
I = $100,000(.05)(1) = $5000
While the above formula helps solve many problems, there are other problems that require another formula. The following formula is fundamental to the relationship between interest, time, present value, and future value:
FV = Future Value = The amount of money to be received or owed at a future date t time periods from now
PV = Present Value = The amount of money to be received or owed at present (i.e., now)
r = Interest Rate = The interest rate on the money, expressed as a decimal
t = Time = The amount of time to pass between PV and FV
Note: The time period, t, and interest rate, r, must be expressed in the same terms. For example, one cannot use an annual interest rate and express time in terms of months. If you are using a value of t that expresses time in months, you must use a monthly interest rate. For more on this topic, see the compound interest section.
The following is an example of a common introductory interest rate problem.
PV = $100,000
r = 5% = 0.05
t = 2
FV = $100,000(1 + .05)2 = $110,250
Types of GMAT Problems
- Expressing Percentages
If X is Y percent of Z then the following arithmetic statement is true:
X/Z=Y/100.What percent of 60 is 25?Correct Answer: C
- The phrasing of the question is difficult for some students. "What percent of 60 is 25?" is the same question as "25 is what percent of 60?" Many students find this later way of phrasing the question easier to work with.
- If X is Y percent of Z, then X/Z=Y/100; For example, 10 is 50% of 20; X = 10, Y = 50, and Z = 20
- Similarly, if you wanted to know what percent of 100 is 50, you would intuitively know that it is 50% since it is 50/100 or 50 per cent (literally, per 100).
- Applying this logic to the problem at hand: 25/60= .42 = 42%
- Thus, 25 is 42% of 60
- If this way of solving the problem is difficult to conceptualize, consider another approach. It should be clear that 30 is 50% (or 1/2) or 60. Since 25 is less than 30, 25 must be less than 50% of 60. This means that any answer that is not less than 50% is wrong.
- Since 25% is 1/4 and 1/4 of 60 is 15 (since 15*4=60), 25% is too small. By process of elimination, the answer is 42%
- Determining the Percent Change
The formula for percent change is:
% Change= (F-I)/I x100
where F is the final value and I is the initial value.A house sold for $500,000 in 1990 and sold ten years later for $400,000. By what percent did the value of the house change?Correct Answer: D
- % Change= (F-I)/I x100, where F is the final value and I is the initial value.
- ($400,000-$500,000)/$500,000= -0.2. A negative value means that the house fell in value. Thus, the value of the house dropped by 20%.
- Comparing Percentages
More advanced problems will require an understanding of percentage (or fraction) comparisons. To solve these, a simple relationship between two fractions must be discovered and solved.In a large forest, 300 deer were caught, tagged, and returned during 2001. During 2002, 500 deer were caught at random, of which only 20 had tags from the previous year. If the percent of deer in the forest that had tags during the second year and were caught in the 500 deer sample is representative of the percent of the total deer population in the forest with tags, what is the total deer population in the forest (assuming no change in population between 2001 and 2002)?Correct Answer: E
- Let N = the total number of deer in the forest.
- During the first year, the percent of deer in the entire population with tags was: 300/N
- 20/500 is the percent of deer caught during the second year that had tags. Since this sample percent matches the percent for the entire population (i.e., the total number of tagged deer divided by the total number of deer), the two ratios are equal.
- Equating these two percents:
Sample = Population
N = (300/1)*(500/20)