The Most Valuable Everyday Math Skill

(via Brett Berry/

Percentages. They come up all the time, in the most casual places: shopping, dining out, grabbing a latte, checking grades, banking, taxes … that makes them the most valuable everyday math skill I can teach you.

I’m proud to say my mom taught me this trick when I was very young. We’d spend Saturdays shopping and she’d quiz me over and over, having me calculate the discounts mentally.

I cannot stress enough how much I want you to master this! The next time you’re out dining, I want you to fill in the tip on your receipt with confidence and gusto and without even one glance at your phone calculator. How’s that sound?

A Little Trick

A percent represents the portion per 100. So 15% means fifteen per one-hundred. Of course most of the time we are not taking a percentage of 100, but of a different value like 10% of 250.

The 10% Trick

To calculate 10% of a number, move the decimal point one position left.

Here are some examples to illustrate:

Why does this work?

Suppose we were calculating 10% of 250 long-hand. I would begin by rewriting 10% as 10/100 and multiplying.

Then I’d reduce 10/100 by canceling out factors of 10.

When dividing by 10 we move the decimal point one place to the left. So 10% of 250 is 25.

Calculating Restaurant Tips

Using the trick we can calculate common tip percentages of 10%, 15% and 20% mentally. Suppose your dinner bill comes to $48.50.

10% Tip Mentally

To find 10%, use the 10% trick.

15% Tip Mentally

To calculate 15% combine 10% and 5% of $48.50. Five percent is half of ten percent, so 5% of $48.50 will be half of 10% of $48.50.

For practical purposes you can approximate the tip, so feel free to round up to $2.50. Finally combine the 10 and 5 percent approximations.

20% Tip Mentally

To find 20% double the 10 percent value.

Calculating Discounts

Another everyday scenario where you might encounter percents is while shopping. For example, suppose we have a sub-total of $168.75. Let’s calculate a variety of possible discounts.

10% off

First take 10 percent of $168.75.

Since it is 10% off, subtract $16.88 from $168.75. An estimate will suit our purposes so round $168.75 and $16.88 to the nearest dollar and subtract.

Our estimation is very close, only 13 cents over the exact answer.

25% off

Now let’s try 25% off. We have two options for finding 25% mentally:

    1. 25% is one-fourth of 100 percent, so we may divide our total by 4.

    2. we may compose 25% by adding two 10%’s and one 5%.

Option One:

Since an estimate is suitable begin by rounding $168.75 to $170 and then use strategic division to divide $170 mentally.

Likewise, $85 ÷ 2 can be split apart and divided individually by 2 to yield $42.50.

Hence the total after discount is approximately $127.50.

Option Two:

Using this method we’ll compose 25% from 10% and 5%. First, approximate 10%.

30% off

To calculate 30% add together three 10%’s. We’ve already approximated 10% of 168.75 as 17, so add three 17’s together.

Therefore 30% off is $51 off and the total after discount is about $119.

50% off

Fifty percent is half off. All we need to do is divide 170 by 2. Therefore the total after discount is $85.

That’s a great start! These techniques will aid you in most percentages you’ll experience day-to-day. 

Take a moment to celebrate some of the amazing achievements from people who had virtually no education at all.

He isn’t the one to let something like being the fourth richest man on the planet stop him from getting a good deal.

There is no Nobel Prize for mathematics, but there are equivalents...