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Percent of a quantity

$p\% = p/100$ ; percent = part ÷ whole × 100.

Watch out: divide the part by the whole, not the reverse.

Percent increase and decrease

New = $(100 \pm p)\%$ of the original; change ÷ original for the percent.

Watch out: the base is the original amount.

Percent error

Error ÷ actual × 100.

Watch out: divide by the actual value, not the measured one.

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