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How to Find the Percent of a Number Mentally

Instructor: Miriam Snare

Miriam has taught middle- and high-school math for over 10 years and has a master's degree in Curriculum and Instruction.

This lesson will teach a few strategies to help you calculate percentages in your head. The lesson includes examples of each method. You will also learn how to extend the strategies to find even more percentages.

Uses of Percents in Daily Life

Have you ever left a restaurant and wondered if you tipped enough because you really just guessed at the amount? Have you ever been surprised by how little you actually saved with a 20%-off coupon? In this lesson, you will learn the tricks to figuring out these and several other common percentages in your head. With a little practice, you will be able to do that quicker than opening the calculator app on your cell phone.

Before we begin, let's do a quick review. In mathematical terms, percent means 'per hundred.' A percent is a portion of the whole and can be converted from a fraction (for example, 25/100 = 25%).

Finding 50% and 25%

The easiest percent to figure out is 50%. You just take half of the number. For example, let's say a sweater used to cost $35 and is now on clearance for 50% off. You just divide 35 by 2 and pay $17.50 for that sweater.

To figure out 25%, you have two options. If the number can be easily divided by 4, then that is the quickest way to find 25%. For example, 25% of 80 is 20 because 80/4 = 20. (Hint: We call 25¢ a quarter because it is 1/4 - read one-quarter - of $1.00).

If you can't divide the number by 4 in your head, the other option is to find 50% of the number and then take half of that answer, because half of 50% is 25%. Let's say we want to know 25% of 7. First, we figure out 50% of 7 by dividing 7 by 2. That gives us 3.5. Then, we have to take half of 3.5. So, 25% of 7 is 1.75.

Finding 10% and 1%

Two more percent values that are very easy to figure out mentally are 10% and 1%. To find 10% of any number, you just need to move the decimal one place to the left. To calculate 1%, you move the decimal two places to the left. So, let's say you want to find 10% of 47. Move the decimal point one place to the left and the answer is 4.7. If you want to find 1% of 47, then you move the decimal two places to the left and the answer is 0.47.

Extending the Strategies

Once you have mastered the tricks mentioned above, there are a few more strategies you can use to find more percentages in your head. For example, if you can find 10% of a number, then you can also find 90% of that number with just one extra step. You just need to subtract the 10% from the original number. So, let's say you need to figure out how many people 90% of 150 people is. You can easily figure out that 10% of 150 is 15. 90% is 10% less than all 150 people. So, figure out 150 - 15, which is 135. Therefore, 90% of 150 is 135.

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