Everybody loves a sale, right? The thrill of the hunt, the sense of accomplishment when landing a great deal.

But how many times have you reached the register and realized your purchase was more than you expected?  Or have you ever passed on a purchase because figuring out the discount was way too much trouble?

You don’t have to be afraid of the mental math that goes along with shopping.  (That goes for in-person and online sales.)  You also don’t have to be that giant geek standing in the sports goods aisle using your cellphone calculator to find 15% of $19.98.  Who has time for that anyway?

Believe it or not, figuring percents is one of the easiest mental math skills.  And it’s one of those things that you may do differently than your sister who may do differently than your boss.  In other words, you are not required to follow the rules that you learned in elementary school.  Now that you’re a grownup, you can find your own way.

Don’t follow?  Let’s look at an example.

Once again you’ve put off buying Mom’s gift.  It’s just about time to leave for her house, and you have literally minutes to find the perfect present for her — at the right price.  You’ve collected $40 from your brother and sister, and you can contribute $20.  Darn it, you’re going to scour the department store until you find something she’ll like that’s in the right price range.

And suddenly, there it is: a countertop seltzer maker, just right for Mom’s nightly sloe gin fizz. Bonus! It’s on sale — 40% off of $89.95.  But can you afford it?

There are a variety of different ways to look at this.  But first, let’s consider what you know.

The seltzer maker is regularly priced at $89.95.

It’s on sale for 40% off.

You can spend up to $60 ($40 from your sibs, plus the 20 bucks that you’re chipping in).

You don’t necessarily need to know exactly what the seltzer maker will cost.  You just need to know if you have enough money to cover the sale price.  And that means an estimate will do just fine.  In other words, finding 40% of $90 (instead of $89.95) is good enough.

Now you have some choices.  You can think of 40% in a variety of ways.

40% is close to 50%

It’s pretty easy to find 50% of $90 — just take half.

50% of $90 is $45

So, if the seltzer maker was 50% off, you could afford it, no problem.  But is 40% off enough of a discount?  You probably need to take a closer look.

40% is a multiple of 10%

It’s not difficult to find 10% of $90 either.  In fact, all you need to do is drop the zero.

10% of $90 is $9

What is 40% of $90?  Well, since 40% is a multiple of 10%:

There are 4 tens in 40 (4 · 10 = 40)

and

10% of $90 is $9

so

 4 · $9 = $36

It’s tempting to think that this is the sale price of the seltzer maker.  Not so fast!  This is what the discount would be.  To find the actual price, you need to do one more step.

$90 – $36 = $54

Looks like you can afford the machine. But there’s an even more direct way to estimate sale price.

40% off is the same as 60% of the original price

When you take 40% off, you’re left with 60%. That’s because

40% + 60% = 100%

Or if you prefer subtraction

100% – 40% = 60%

So you can estimate the sale price in one fell swoop.  Like 40%, 60% is a multiple of 10%.

There are 6 tens in 60 (6 · 10 = 60)

and

10% of $90 is $9

so

 6 · $9 = $54

The estimated sale price is $54, which is less than $60.  You snatch up the race-car red model and head for the checkout.

There are so many other ways to estimate sales prices using percents.  Do you look at these differently?  Share how you would estimate the sale price in the comments section.

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