Like all math calculations, you can work out the percentage of change with a formula. If you receive a raise at work, you probably want to know by what percentage your pay increased. Or maybe the value of your home has decreased and you want to calculate the percentage of decrease. Many daily situations arise when it might be handy to be able to measure the percentage of change.

Decrease is calculated in the same manner. In the drive-time example, if it took you 30 minutes to drive to work today and 45 minutes yesterday, the calculation would be as follows: 30 minus 45 equals a change of -15; -15 divided by 45 equals -.33; -.33 times 100 equals -33. Your drive time decreased by 33 percent.

Determine the amount of change in the numbers. Subtract the old amount from the new amount. For example, if it took you 45 minutes to drive to work today and 30 minutes to drive to work yesterday, subtract 30 from 45 to obtain a change of 15 minutes.

Divide the amount of change by the original amount. In our example, divide 15 by 30 to obtain .5.

Multiply by 100 to convert the result to a percentage. When you multiply .5 by 100, the result is 50. Therefore, your drive time increased by 50 percent from yesterday to today.

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