Thirty percent of 40 equals 12. This can be calculated by multiplying 30/100 x 40. It can also be found by multiplying 0.3 and 40. In both instances, the answer is 12.

Percentages are important in math and science. Often, percentage calculations must be completed from word problems. In this instance, the word "of" always indicates multiplication. For this problem, 30 percent of 40 equals 30/100 multiplied by 40.

The first step of percentage problems is to always convert the percentage to a decimal point or a fraction. In this instance, 30 percent can be converted into 30/100 or 0.3. Once the percentage has been converted, multiply the decimal or fraction by the whole amount to determine the final percentage.