Percentages

It's amazing how many college students struggle with percentages. Percent just means "per hundred". It's a simple way of comparing parts of a whole; that is, fractions.

## Converting a Fraction to a Percentage

Just multiply the fraction by 100%.

Example: if you and two friends share a pizza equally, you get 1/3 of the pie, or 1/3 x 100% = 33.3%

What if for some crazy reason someone offered you 5/17 of the pie instead? Percentages make it easy to compare fractions.

Here the percentage is 5/17 x 100% = 29.4%. You can now see easily that that's less than 33%, so you would choose the third instead.

## Calculating a Percentage Increase

Percentage increase formula: percentage increase = (new number - old number)/old number x 100%

Example of percentage increase calculation: if you were making \$8 an hour, and your pay increased to \$11 an hour, the percentage increase would be

(new number - old number)/old number x 100% = (\$11-\$8)/\$8 x 100% = 37.5% increase

Students often get confused when a number more than doubles, but the same formula applies.

Example: the U.S. budget deficit increases from \$1.2 Trillion to \$4 Trillion. What is the percentage increase?

The percentage increase is (new number - old number)/old number x 100% = (\$4 Trillion-\$1.2 Trillion)/\$1.2 Trillion x 100% = 233.3% increase.

Note than a 100% increase means that something doubles. A 200% increase means that the quantity tripled. In the previous example the budget deficit more than tripled.

page revision: 2, last edited: 20 Dec 2011 22:47