A percentage figure represents the number of parts per one hundred a portion amount corresponds to. For example, "75 percent" is another way of saying "75 parts per 100." To calculate a percentage, the whole amount must be known, in addition to the percentage or portion amount. The question may be "what percentage of W is P," where W is the whole amount and P is the portion amount. Or the question may be "how much is X percent of W," where X represents a percentage figure.
Whole Number and Portion Amount Known

Write down the whole number amount.
For example: 500.

Write down the portion number amount as a numerator over the whole number amount.
For example: 120 / 500.

Calculate the equation. The result is the decimal equivalent of the percentage.
For example: 120 / 500 = 0.24.

Multiply the decimal equivalent of the percentage by 100; or move the number's decimal point over two places to the right. The result is the percentage amount.
For example: 0.24 * 100 = 24 (Therefore in answer to the question "120 is what percent of 500," it is learned that 120 is 24 percent of 500).
Whole Number and Percentage Known

Write down the percentage amount.
For example: 24 percent.

Divide the percentage amount by 100, or move the number's decimal point over two places to the left, to convert it to its decimal equivalent.
For example: 24 / 100 = 0.24.

Write down the whole number amount.
For example: 500.

Multiply the whole number amount by the decimal equivalent of the percentage. The result is the percentage portion amount.
For example: 500 * 0.24 = 120 (Therefore in answer to the question "How much is 24 percent of 500", it is learned that 24 percent of 500 is 120).
Whole Number Amount and Percentage Known

Enter the whole number amount in the calculator.
For example: enter 500.

Press the times key.

Enter the percentage amount.
For example: enter 24.

Press the percent key. The result is the percentage portion amount (should be 120).
References
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