How to Use Percentages to Easily Calculate Sales Prices

Learn how to use the power of ten percent to quickly and easily calculate how much money a sale will save you.

Jason Marshall, PhD
4-minute read
Episode #13

How to Round Numbers to Make Estimating Savings Easier

Okay, that’s easy enough. But before we get too excited and run off on a celebratory shopping spree, let’s think about one more problem. This time you’re interested in a shirt on sale for 40% off its normal price of $28. What’s the final price after applying the 40% discount? You could start, as usual, by finding 10% of the initial price—which would be $2.80—and then proceed to calculating the full 40% discount by multiplying this 10% discount by 4. And, if you need a precise answer, that’s a great thing to do. In this case, 4 x $2.80 = $11.20, so the precise final price is $28.00 - $11.20 = $16.80.

But, to be honest, that’s a bit more work than I prefer since I’m usually perfectly content to know only approximately how much the shirt is going to cost me. So, let’s instead choose to work smarter and save ourselves some of the unnecessary effort. After all, unlike many areas of life, doing less work in math isn’t a sign of laziness, but is instead a sign of clever thinking! So, here’s the quick and dirty tip: Let’s pretend the shirt in question isn’t really selling for $28, but is instead selling for $30. In other words, let’s round the price up to the nearest ten dollars.

How does that help? Well, it helps because calculating 40% of $30 (10% of $30 is $3, so 40% of $30 is 4 x $3 = $12) is much faster to do in your head than calculating 40% of $28 (which we did earlier). So back to the initial question: What’s the final discounted price? Well, it’s the initial price minus the discount: $30 - $12 = $18, right? No, not exactly since this was just an estimate. Remember, the actual final price we calculated before was $16.80. But our quick and dirty estimate of $18 isn’t off by much—and it’s a lot faster too. Just remember to keep in mind whether you’re dealing with precise or estimated values to make sure you’re not surprised at the register!


About the Author

Jason Marshall, PhD

Jason Marshall is the author of The Math Dude's Quick and Dirty Guide to Algebra. He provides clear explanations of math terms and principles, and his simple tricks for solving basic algebra problems will have even the most math-phobic person looking forward to working out whatever math problem comes their way.