If there were one thing that had to rank as important when it came to practical mathematics it would be the ability to calculate a percent in your head. Just think of those shopping sprees when merchants were discounting their wares. You are in and out of the different stores, each posting their signs indicating 20% off, 30% off, and 40% off. It would be nice to be able to calculate your savings quickly and accurately. Here we learn how.
Calculating with Percents
The way to calculate with percentages is quite easy and only depends on three units of percents and then taking appropriate multiples of these units. The units are 10%, half of this or 5%, and one-tenth of 10% or 1%. The way this works is quite simple and is based on finding 10% or 1% of a number by shifting the decimal point to the left. We can then take appropriate multiples to get, for example, 20% or 2%, or by taking half of 10%, 5%. Let us examine all this with specific examples.
Suppose you are at the mall and the shirt that you wanted last week that cost $50 is no on sale. The store is giving a 10% discount. To find 10% of a number, simply shift the decimal point one place to the left. In a number such as 50 (we will remove the $ sign for now), the decimal point is understood to be to the immediate right of the last digit on the right, in this case 0; thus we can write 50 as 50.0 to show the "unseen" decimal point.
To calculate 10% of 50.0, we move the decimal one place to the left to get 5.0, or simply 5, as we can now do away with the decimal point. If you wish to know the price of the shirt after the discount, you only need subtract this computed 10% or $5 from the original $50 to get $45.
More Involved Percent Calculations Made Easy
Now that you know how to calculate 10% of any number, it is very easy to calculate 5%, by taking half of 10%, or 1% by shifting the decimal point two places to the left. Let us examine each of these. Take the example before. The original price is $50. Suppose the store is offering a 5% discount off the shirt because it is "bonus Tuesday." To get 5%, simply take half of 10%. We saw how to get 10% using a decimal shift. The result is 5. Now half of 5 is 2.50. Thus the shirt will cost $50 - $2.50 or $47.50 on "bonus Tuesday."
If you are thinking ahead, then you can start to see how easy it can be to work with percents and to calculate discounts when shopping. Suppose the store is offering a 10% discount on all purchases and an extra 1% off if you spend more than $100. You buy a shirt for $48, a pair of jeans for $57, and a belt for $15. The total is $120 and you will receive not 10% but 11% off your purchase. How do we figure this? Quite easy. First find 10% of 120.0 (notice we have added the decimal point for clarity). This is 12. Then find 1% of 120.0 by shifting twice to the left. This is 1.20. Put the two results together to get 13.20. Thus you will save $13.20 and pay $106.80.
By using various combinations of 10%, 1%, and 5%, you can get almost any percent needed quickly. By using this method, you can even get 1/2% by taking one half of 1%. What more is needed? Try this method out the next time you go shopping and be sure that you get the great deals you so rightfully deserve. And who knows? You might have so much fun with this technique that you only go shopping when you get those sweet percentages off. What a nice thing for your budget!
References
- Fisher, Richard W. Decimals and Percents: Mastering Essential Math Skills. Self-published, 2008.
- Fosnet, Catherine Twomey and Maarten Dolk. Young Mathematicians at Work: Constructing Fractions, Decimals, and Percents. New Hampshire: Heinemann, 2002.
Joe Pagano - Joe is a prolific writer, author, poet, and linguist, who has published over 200 articles and several books on a broad array of ...
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