**Newsroom Math Crib
Sheet**

By Prof. Steve Doig

Arizona State University

**To convert a fraction into a
decimal:**

n
Divide the top number by the bottom number

n
Examples: 5/8 = 0.625 17/64 =
0.265…

**To convert a decimal into a
percentage:**

n
Multiply by 100 (or simply move the decimal two places to the RIGHT)

n
Examples: 0.658 = 65.8% 1.255
= 125.5%

**To turn a percentage into a
decimal:**

n
Divide by 100 (or simply move the decimal two places to the LEFT)

n
Examples: 43.7% = 0.437 148.2% = 1.482

**To get X% of Y:**

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Turn X% into a decimal, then *multiply*
it by Y

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Example: 20% of 90 = 0.20 * 90 = 18 130.5% of 45 = 1.305 * 45 = 58.7…

**To compare X and Y using
percentages (X is what percent of Y?):**

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X is (X/Y * 100) percent of Y

n
Example: 5 and 8: 5/8 = .625 =
62.5%, so 5 is 62.5% of 8

n
Example: 8 and 5: 8/5 = 1.6 =
160%, so 8 is 160% of 5

**To compare X and Y using
percentage differences:**

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X is ((X/Y –1) * 100) MORE/LESS than Y

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Use MORE THAN if the answer is positive, and LESS THAN if it’s negative

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Example: 5 and 8: 5/8 –1 = .625 – 1 = -0.375 = -37.5%, so 5 is 37.5%
less than 8

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Example: 8 and 5: 8/5 –1 = 1.6 – 1 = .6 = 60%, so 8 is 60% more than 5

**To compare a NEW number with
an OLD number using percentage change:**

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NEW has increased/decreased ((NEW/OLD –1) * 100) percent since OLD

n
Use INCREASED if the answer is positive, and DECREASED if it’s negative

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Example: This year’s $8 million budget is a 60% increase over last
year’s $5 million budget.

n
Example: This year’s $5 million budget is a 37.5% decrease from last
year’s $8 million budget.

**To calculate rates (the
number of events per some standard unit):**

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Do this to account for different size populations

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RATE = (EVENTS / POPULATION ) * (“PER” Unit)

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Example Problem: If there were 320 murders in a population of
1,937,086, what is the murder rate per 100,000?

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First, divide the 320 murders by 1937086 = 0.0001652…

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Now multiply 0.0001652… by 100,000 = 16.5 murders per 100,000
population

**To calculate the effect of
inflation using the Consumer Price Index (CPI):**

__Price Now__ _{=} __CPI Now__

Price Then CPI Then

n
With this formula, all you need is any three of the numbers to
calculate the fourth.

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Example: CPI now = 194.6; CPI in 1965 was 30.8; price of gas in 1965
was $0.30 per gallon.

X / 0.30 =194.6 / 30.8

X = (194.6 / 30.8) * 0.30 = 6.32 * 0.30 = gas in
1965 cost the equivalent of $1.90 per gallon

**Newsroom statistics:**

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Mean (average): Add the numbers, then divide by how many numbers there
are

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Median: Sort the numbers in order, then find the middle value

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Sampling error margin: 1/ÖN (example: sample of 625: 1/Ö625 = 1/25 = 0.04 = +/- 4
points)

*"Ratios, Percentages, Statistics! Oh My!"*

News
Division Program, SLA 2005 Annual
Conference

To request updates or additions contact Steve Doig

Last updated 6/20/2005