Calculating Percentages

Percents are all around us in real life. Going out to eat and having to tip the waitress. Going shopping and seeing deals such as 25% off. Sales tax. Without a knowledge about what percents mean and how to find them will put you at risk for people to cheat you out of your hard earned money.


So what is a percent? The word percent can be literally broken down into “per 100.” A percent is the part out of the whole. I will help you calculate finding a percent, finding the number of a percent, and finding percent increase/decrease.



  1. Finding a Percent of the Whole

If you are trying to find the percent of women in the room, you would need to take the number of women and divide that by the number of people in the room and multiply by 100. Lets say there are 4 women and 6 men in a room. We will take 4/10*100=40%. But how did I get 10 in that equation? Well I need the number of people in the room so I had to add the 4 women and 6 men together to get 10.


The equation to find any percent is

(Part / Whole) * 100 = Percent


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  1. Finding the Number of a Percent

This method is useful for when you need to calculate the tax or a tip. Both of those are set percents.


If I rearrange the equation: (Part / Whole) * 100 = Percent to solve for the Part, we get (Percent/100)*Whole = Part. In order for this equation to work, we need to get the percent to a decimal. That is why we take the percent and divide by 100. That will turn the percent to a decimal. So lets say we want to tip 20% on a 50 dollar food bill.


I need to take (20%/100) *50 =10. We would need to tip 10 dollars. To do this in your head, if I divide anything by 100, I move the decimal two spots to the left. So 20% becomes .20. Then I can take .2*50 = 10.


That will work for any set percent when you know the total.


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  1. Finding Percent Increase/Decrease

When I go out shopping, I am always looking for the best deals! So how do I know what the best deal is? 25 dollars off or save 25%? If you do not know how to find the percent decrease, you could be wasting your money. Lets see how to find a percent increase/decrease.


Its like the first example, but we have to add another step. We first need to find the amount of the percent. Then if it is a decrease we have to subtract that amount from the original. If it is an increase, we will have to add it to the original amount.


Lets look at the example 25% off. Because it says 25% off, we know it is a percent decrease. The sweater we want is originally $40. So we need to find 50% of the 40.


(25/100)*40 = 10. So our percent is 10. Then we need to subtract that from the original 40. 40-10= 30. The final price of the sweater is $30.


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