Percent of a Number
Part of the Decimals, Percents & Ratios section of Coddy's Pre-Algebra journey. Lesson 27 of 56.
What is 20% of 45? In mathematics, of means multiply, so 20% of 45 is 20% × 45. On the board, a percent is a number like any other, so you can write it exactly like that.
To work it out, write the percent as a fraction over 100: (20/100) × 45. Now multiply the top by 45 and divide by 100: 20 times 45 is 900, and 900 divided by 100 is 9. So 20% of 45 is 9.
The answer makes sense: 20% is a fifth of the whole, and a fifth of 45 is 9. Checking a percent against a simple fraction like this, a half, a quarter or a tenth, catches most slips before you press Check.
Challenge
Work out 15% of 80, which the board writes as 15% × 80. Write the percent over 100 first, then finish.
Try it yourself
Simplify the expression
Test yourselfThis lesson includes a short quiz. Start the lesson to answer it and track your progress.
All lessons in Decimals, Percents & Ratios
4Dividing Decimals
Dividing by a DecimalDividing by 10 and 100Dividing by a Whole NumberUnit PriceRecap: Dividing2Adding Decimals
Adding DecimalsSubtracting DecimalsNegative DecimalsMaking ChangeRecap: Add and Subtract3Multiplying Decimals
Multiplying DecimalsTimes 10, 100, 1000Bigger ProductsBuying by WeightRecap: Multiplying6Percent Basics
What Percent MeansPercent of a NumberPercent as a DecimalWhat Percent Is ItRecap: Percent Basics