Calculating the sale price of an item is one of the few math problems that most people will actually use after high school. Nevertheless, many of us don’t know how to do it. Not knowing what the price will be at checkout is frustrating. And not knowing how to help customers is even more frustrating. Thankfully, there are some tricks to make it easier.
Discounts are usually given as a percentage off an original price. So the trick to finding a discount is in knowing how to find the percentage of a number. Thankfully, again, sale prices are given as round numbers so they’re not too hard to figure out.
There are two ways to find a percent: with a calculator or with your brain. Neither is difficult. You just have to know the procedure.
Finding a Discount With a Calculator
There are three steps to finding the sale price. (I’m not going to bother you with fractions. That’s probably what made you confused back in high school.)
Procedure
- In the percent, move the decimal point two places to the left.
- Multiply the original price by the decimal to get the discount.
- Sale price = Original price – discount
Where is the decimal point?
What you need to know about numbers is that 20 is the same as 20 with a decimal at the end of it. Any number can be written this way, so 60 is the same as 60. and 10 is the same as 10. and 5 is the same as 5. and so on. Once you know that, moving the decimal two places to the left is easy. If you forget which way to move the decimal, just remember that we need to make this number smaller, not larger, or you’ll get some ridiculously huge discount that you just know can’t be right.
Example
What is 20% off of $34.88?
Start by writing the percent as a decimal. Then, move the decimal two places to the left to get .20
Now, multiply that by the original price to find the discount.
(34.88)(.20) = 6.97
34.88 – 6.97 = 27.91
Example
What will the sale price be if an item that costs 105.50 is marked down 35%?
Writing the percent as a decimal we get .35
Multiply that by the original price to get 36.92
105.50 – 36.92 = 68.58
Example
Take 5% off of $75.
Move the decimal two places to the left to get .05
(75)(.05) = 3.75
75 – 3.75 = 71.25
Finding a Discount In Your Head
If you have to do this in your head, the procedure is a little different. We will always work with 10 percents as much as we can because finding 10% is easy. To find 20 or 30 percent you will just take 10 percent 2 or 3 times.
How to find 10% of a number in your head
To find 10% of any number, just move the decimal one place to the left of that number. This new number is 10% of the old one.
To make it really easy, you can round the original price to the nearest dollar. So 71.89 becomes 71.
Example
To find 10% of $20.50 ignore the change. Move the decimal point one place to the left to get $2. Subtract that off the original price and your sale price is about $18.
How to find 20% of a number in your head
If a sale were 20% off, you’d just take 10% twice. 10%+10%=20%
Example
If a $65.00 item is 20% off we can find the discount with our 10% trick. So moving the decimal one place to the left we get $6.50. This is the 10% discount.
20% is twice that so that’s 6.50 + 6.50 = 13. The 20% discount is $13.
Your sale price will be 65 -13 = 52.
How to find 35% percent of a number in your head
You can still use the 10% trick with this but for the other 5%, you’ll cut 10% in half.
So you’ll be adding 10% + 10% + 10% + 5%
Example
What is 35% off $80?
First we find 10% by moving the decimal point of $80.00 one place to the left to get $8. Now, to find 30% we take that $8 three times. 8 + 8 + 8 = 24. But we need to find 35% and not 30% right? To find 5% you just need to cut 10% in half. This gives us $4.
So, 35% of $80 is 8 + 8 + 8 + 4 = 28. The discount is $28 and the sale price then is 80 – 28 = 52.
