In the previous two posts, we have learned two types of calculations about percentage: ignoring the percent sign and making use of 10%. In this post, we learn another math shortcut that is very useful in speed calculations about percentage.
One of the easiest ways to calculate for a percentage of a number is to convert percent into fractions. For example, 50% is equivalent to 1/2 so instead of multiplying the number by 0.5, you can just multiply by one half (or equivalently) divide by 2. Here are some examples.
What is 50% of 284?
Fifty percent is the same as one-half, so we just calculate for half of 284 which can be done mentally. Half of 284 is 142.
What is 25% of 800?
Twenty five percent is equal to 1/4 in fraction so, we just get 1/4 of 800 which equals 200.
What is 75% of 480?
The equivalent of 75% in fraction is 3/4, so we just have to get 3/4 of 480. That is, we divide 480 by 4 which results to 120, then multiply by 3 which results to 360.
A $60 t-shirt has a 20% discount. How much is the discounted price?
Twenty percent is equivalent to 1/5. So, 1/5 of $60 (that is $60 divided by 5) is $12. So, the discounted price is $60 – $12 = $48.
In this example, another short cute maybe done using decimals. Since the discount is 20%, only 80% of the price will be paid. Eighty percent is the same as 0.8 and if we multiply it by $60 we get $48.
To be able to do this effectively, you have to memorize the equivalent fractions of common percentage use in calculations and discounts. Here are some of them:
10% = 1/10
20% = 1/5
40% = 2/5
25% = 1/4
50% = 1/2
60% = 3/5
75% = 3/4
80% = 4/5
So, that’s it for now for our percentage trick series, we will be learning more math tricks and short cuts which will make our calculations easier in the next posts.