Math Tools for Journalists

By Daniel Temple

Nov. 3, 2008


Chapter 2 Summary- Percentages


            This chapter basically goes into percentage and the kinds of applications of percentages that a journalist might need to know.  It goes into specifically: Percentage increase, percentage decrease, and percentage of the whole and percentage points.

            In order to calculate a percentage increase or decrease, you simply take the original figure and subtract it from the new figure.  You then divide this number by the old figure.  You then convert this number to a percentage by moving the decimal point two places to the right.   This will give you the percentage increase or decrease.

Often times, journalists will have to know certain percentage applications in regards to salaries.    These are a couple useful formulas. 

* Original salary x percentage increase= dollar amount of salary increase for the first year

* Original salary + salary increase= salary for the first year of the contract

* First year salary x percentage increase= dollar amount of salary increase for second year

*First year salary+ salary increase= salary for the second year

Percentages of the whole are useful because they can often put large numbers into perspective.  In order to calculate the percentage of a whole you simply divide the subgroup by the whole group and then move the decimal point two places to the right.

When working with numbers that are already percentages, says Wickham, it’s important to distinguish between percent and percentage point. 

The chapter also went into a couple of baseball statistics that are important to know if you’re covering baseball.  It gave the methods for calculating batting average, slugging percentage an earned run average.

Wickham also talks about the importance of simple interest and the formula for calculating it.  In order to calculate interest, one simply multiplies the principal (amount of money borrowed) by the rate (as a decimal) by the time (in years). 

“Compounding”, according to Wickham, means the interest is added to the original principal and subsequent compoundings. 

Compounds are used primarily for figuring out loans and how much is owed after interest.  Often times consumers will make monthly payments on their car or home.  The formula for calculating this monthly compounded interest is as follows.

A= monthly payment

P= original loan amount

R= interest rate, expressed as a decimal and divided by 12

N= total number of months


A= (P x (1+R) (to the Nth power) x R) / (1+R) (to the Nth power) -1)\


            Wickham closes by talking about savings account and how to calculate savings deposits after a certain amount of time.


B= balance after one year

P= principal

R= interest rate

T= number of times per year the interest is compounded


B= P (1+ (R/T)) (to the power of T)





4 Practice Questions


1.     The Madison City Council budgeted $5,283 for snow removal this year.  Last year, Madison budgeted $14,700 for snow removal.  What is the percentage decrease?

       64% decrease


2.     The mayor of Oakville announces today that 4/5 of the village’s streets were cleared within a day of a major snowstorm.  What percentage of the streets was cleared?



3.     Jack Garfield is a patrolman for the Henderson Police Department.  He scored a within the 60th percentile on the sergeant’s exam.  He knows that 116 patrolmen took the test.  Only the top 30 scorers will be promoted to sergeant.  Did he make it?

       No he did not, you have to score within the 75th percentile in order to be promoted


4.     The CPI in July 1990 was 130.4.  It increased to 131. 6 by August 1990.  What was the percentage increase in CPI for that month?


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