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A book club had 60 members last year.

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This year, it has 48 members.

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Determine the percentage decrease in the number of club members.

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A percentage decrease is equal to the amount of the decrease divided by the original amount multiplied by 100.

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The first part of this calculation gives the proportion of the original amount that the decrease represents.

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The second part converts it from a decimal to a percentage.

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Let’s look at the numbers in this question.

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The book club had 60 members last year.

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So the original amount is equal to 60.

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The book club now has only 48 members.

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Therefore, the amount of the decrease is equal to the difference between these two values: 60 minus 48 which is 12.

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Substituting each of these values into the calculation gives that the percentage decrease is equal to 12 over 60 multiplied by 100.

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12 and 60 both share a factor of 12 and so they can be cancelled by this amount, giving one in the numerator and five in the denominator.

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The calculation therefore simplifies to 100 over five, which is equal to 20.

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The percentage decrease is 20 percent.

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Now, there is a common error to watch out for when calculating percentage increase or decrease.

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Remember that the formula for percentage decrease tells us that we should be dividing by the original amount.

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The common error is to divide instead by the new amount, which in this case would be 48, giving an answer of 25 percent.

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Watch out for this when you are answering questions about percentage change.

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The amount you divide by must be the original amount, not the new amount.