Learn How to Calculate Sales Tax

One of the most common types of percent problems you'll encounter in daily life is calculating sales tax. It's not hard to do. Here's an example of how a student worked through percent and sales tax problems and tips on how you can learn to master the technique, too.

I was tutoring Jason (not his real name), to prepare him for Algebra. He attended tutoring on the Internet with the help of his high-speed internet connection, web camera, computer, and graphing calculator. Thankfully, all of the technology was functioning properly and we were on our way to higher grades.
 
"Today," I began, "we are going to review percents and sales tax."
"Ok, Ms. Jennifer, I got this. I know all about sales tax." Jason confidently announced as he pulled out a pen.
"Oh no, Jason. Where is your pencil?"
"Pencil?" Jason was puzzled about a ten cent pencil, yet he sat in a maze of hundreds of dollars of gadgets.
"Yes, Jason, a pencil. You know that we do not do math in pen."
"Yes ma'am."
 
Jason hunted down a pencil and sharpened it with a butter knife. A hand sanitizer gel spill had ruined his pencil sharpener but rendered it 99.9% germ-free.

After Jason put away his primitive sharpening tool, we talked about a printer that he was planning to buy. At $125, the printer was a bargain, but I emphasized that he needed to know the true amount to stay within his budget. If the sales tax rate was 8%, then how much would he pay in sales tax for the printer?

What do you know?
The sales tax rate is 8% or 8 percent. Realize that 8 percent means 8 per 100.

8% = 8/100
The printer cost $125.00

With percents, think part/whole.
8 (part)/100 whole = x (the part, or the sales tax amount is unknown) /125 (the whole)
8/100 = x/125

Cross multiply. Hint: Write these fractions vertically to get the full understanding of cross multiplying. To cross multiply, take the first fraction's numerator and multiply it by the second fraction's denominator. Then take the second fraction's numerator and multiply it by the first fraction's denominator.

8 * 125 = x * 100
1000 = 100x

Divide both sides of the equation by 100 to solve for x.
1000/100 = 100x/100
10 = x

Verify the answer.
Does 8/100 = 10/125
8/100 = .08
10/125 = .08

So, he would spend $135 ($125 + $10) on a $125 printer.

Note: Add $125 and $8 to get the total amount. Remember, the sales tax is 8% of the price, not $8.

Original Worksheet

1. Laptop Bag
Price: $18
Sales tax rate: 9%
Sales tax amount:$1.62
Final cost: $19.62

What do you know?
9/100 = x/18

Cross Multiply and Solve.

9 * 18 = x * 100
162 = 100x
162/100 = 100x/100
$1.62 = x

Verify the Answer.

Does 9/100 = 1.62/18?
9/100 = .09
1.62/18 = .09

$1.62 + $18 = $19.62

2. Anti-Virus Software
Price: $50
Sales tax rate:  8.25%
Sales tax amount: $4.125
Final cost: $54.13

What do you know?
8.25/100 = x/50

Cross Multiply and Solve.

8.25 * 50  = x * 100
412.50 = 100x
412.50/100 = 100x/100
$4.125 = x

Verify the Answer.

Does 8.25/100 = 4.125/50?
8.25/100= .0825
4.125/50= .0825

$4.125 + $50 ≈ $54.13

3. USB Drive
Price: $12.50
Sales tax rate: 8.5%  
Sales tax amount: $1.0625
Final cost: $13.56

What do you know?
8.5/100 = x/12.50

Cross Multiply and Solve.

8.5 * 12.50  = x * 100
106.25 = 100x
106.25/100 = 100x/100
$1.0625 = x

Verify the Answer.

Does 8.5/100 = 1.0625/12.50?
8.5/100= .085
1.0625/12.50 = .085

$12.50 + $1.0625 ≈ $13.56

4. Graphing Calculator
Price: $95
Sales tax rate:  6%
Sales tax amount: $5.70
Final cost: $100.70

What do you know?
6/100 = x/95

Cross Multiply and Solve.

6 * 95  = x * 100
570 = 100x
570/100 = 100x/100
$5.70= x

Verify the Answer.

Does 6/100 = 5.70/95?
6/100= .06
5.70/95= .06

$95 + $5.70 = 100.70

5. MP3 Player
Price $76
Sales tax rate:  10%
Sales tax amount: $7.60
Final cost: $83.60

What do you know?
10/100 = x/76

Cross Multiply and Solve.

10 * 76  = x * 100
760 = 100x
760/100 = 100x/100
$7.60 = x

Verify the Answer.

Does 10/100 = 7.60/76?
10/100= .10
7.60/76 = .10

$76 + $7.60 = $83.60

6. Laptop Computer
Price: $640
Sales tax rate:  8.5%
Sales tax amount: $54.40
Final cost: $694.40

What do you know?
8.5/100 = x/640

Cross Multiply and Solve.

8.5 * 640  = x * 100
5440 = 100x
5440/100 = 100x/100
$54.40 = x

Verify the Answer.

Does 8.5/100 = 54.40/640?
8.5/100= .085
54.40/640 = .085

$640 + $54.40 = $694.40