Have you noticed that the school supply aisle is turning into the janitor's closet? Lysol disinfectant, paper towels, Clorox wipes - these toiletries are the hallmarks of a hospital, not a 2^{nd} grader's Spongebob Squarepants backpack. For older students, laptops, smart phones, and Kindles are becoming unofficial school supplies. What happened to number 2 pencils? I'll tell you. They're collecting dust in a junk drawer.

### Sharp Mind

Last spring, 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.

### Calculating Sales Tax

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 = 100*x*

Divide both sides of the equation by 100 to solve for *x*.

1000/100 = 100*x*/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: Do not add $125 and $8 to get the total amount. Remember, the sales tax is 8% of the price, not $8.*

### Answers and Explanations

**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 = 100*x*

162/100 = 100*x*/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 = 100*x*

412.50/100 = 100*x*/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 = 100*x*

106.25/100 = 100*x*/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 = 100*x*

570/100 = 100*x*/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 = 100*x*

760/100 = 100*x*/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 = 100*x*

5440/100 = 100*x*/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