Discounts and interests are some of the practical application of percentage. We often see 15% discount in the things we buy in stores and we also hear interests of money on loans. But how can we calculate percentage easily? Here are some math tricks on calculating percentage.
Percentage Math Trick 1: Ignoring the Percent Sign
The first strategy is to ignore the percentage sign, multiply the two numbers. And then, after getting the product, divide it by 100. Dividing by 100 is so easy, just move the decimal point to digits to the left or if there are two zeroes or more on the right, just eliminate the two zeroes.
What is 20% of 300?
Well, just multiply 20 by 300 which gives 6000, then divide by 100. Dividing by 100 mentally is very easy because you just remove the two zeroes. So, 6000 becomes 60. So, 20% of 300 is 60.
What is 15% of 200?
Again, we ignore the precent sign and then multiply first. We have
15 × 200 = 6000.
Then, we divide 600 by 100 or move the decimal places two digits to the left which becomes 60.
A $400 tablet is on sale! It has a 12.5% discount. How much can you save if you buy it?
We multiply the numbers ignoring the percentage sign.
12.5 × 400 = 5000
We divide 5000 by 100 or eliminate two zeroes. This gives us 50.
Why the Percentage Math Trick Works
The common method taught in schools to get the percentage of a number is to change the percent to decimal first which means that we have to divide by 100. So, if the percentage is p, then to get the decimal, we divide p by 100. That is
We then multiply the result by n giving us
Note that the expression above is the same as
What we did in the trick is we multiply first the number and the percentage ignoring the sign, which means that we multiply p and n first,
and then divide the result to 100 to become
As you can see, the results are the same therefore, we can do this trick to any number.
In the next post, we are going to learn a more interesting math trick about percentage.
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