Twenty percent of 100 is 20. To calculate the percentage of a number, convert the percentage to a decimal number; move the decimal point two places to the left, and multiply by the number. The word percent means per 100.
While this example requires simple math, other problems involve more complex number manipulations. Percentages are also expressions of ratios, so 20 percent is also written as 20/100, a fraction that simplifies to 1/5.
While it is common to see the percent sign in science and math documents, writing style guides indicate using the word percent as their preference. Writing style guides also indicate that when a percent is not a whole number, the proper form is to write the number as a decimal rather than a mixed number.