Course Catalog / BMU 020 / Lesson 2 of 5
BMU 020
Lesson 2: Percents, sales, tips, and tax
Quick review
1. What is a unit price?
See the answer
The cost for one unit, like one ounce.
2. How do you find it?
See the answer
Price divided by size.
Why this matters
Signs say 25 percent off. A bill asks if you want to add a tip. A receipt adds tax. All of these use percents.
Percent means out of 100. In this lesson you will learn a few fast tricks and how to check them on your phone.
Start with what you already do
You already know half. Half of $10 is $5. That is 50 percent.
Common belief: I need a lot of math to work with percents.
What is actually true: A few tricks cover most of what you need. Ten percent is the key that opens the rest.
In this lesson you will
- Find 10 percent of a price in your head
- Work out a sale price
- Work out a tip and sales tax
Key words
- percent (%)
- Out of 100. 25 percent means 25 out of every 100.
- discount
- The amount taken off a price.
- sales tax
- Money added to a price for the state or city. It is often a percent.
The 10 percent trick
To find 10 percent, move the decimal point one place to the left. 10 percent of $40.00 is $4.00. 10 percent of $36.50 is $3.65.
| Percent | Fast trick | Example with $40 |
|---|---|---|
| 10 percent | Move the decimal one place left | $40 becomes $4 |
| 20 percent | Find 10 percent, then double it | $4 times 2 is $8 |
| 25 percent | Divide by 4 | $40 ÷ 4 is $10 |
| 50 percent | Divide by 2 | $40 ÷ 2 is $20 |
To check on a phone, change the percent to a decimal. 25 percent is 0.25. Type the price, tap times, then type 0.25.
Sale prices
A sale takes a percent off. Find the discount, then subtract it from the price. A $40 shirt at 25 percent off has a $10 discount. You pay $30.
Under FTC pricing guides, a was price should be a real price the store openly offered for a fair amount of time. A fake high was price makes the sale look better than it is. Check the unit price too. The NIST unit pricing guide says a sale tag should show its own unit price, as clear as the regular one.
Tips
Many people tip a percent of the bill. Pick the percent you want. Your phone calculator can check your work. For 20 percent on a $36 bill, find 10 percent, $3.60, then double it. The tip is $7.20.
Sales tax
Many places add sales tax at the register. The rate depends on where you live, and some items may not be taxed. Look for the rate on a receipt. If you cannot find it, ask the store. For 8 percent tax on $30, type 30 times 0.08. The tax is $2.40, so you pay $32.40.
Text version of this figure
Percents made simple. Start with 10 percent.
- Step 1, Find 10 percent: Move the decimal one place left.
- Step 2, Build from it: Double for 20, half of 10 for 5.
- Sale price: Price minus the discount.
- Tip or tax: Price times the percent as a decimal.
- Check on your phone: 25 percent is 0.25. 8 percent is 0.08.
Source: FTC Guides Against Deceptive Pricing, 16 CFR 233.1 (Oct 29, 2026).
Worked example: Bree shops a sale
Bree sees a $60 jacket at 20 percent off. Tax where she lives is 6 percent.
- 10 percent of $60 is $6. Double it: the discount is $12.
- Sale price: $60 minus $12 is $48.
- Tax: 48 times 0.06 is $2.88.
- Total: $48 plus $2.88 is $50.88.
Bree is a made up example. Tax rates are different from place to place.
Practice: Percents at the store
Use the 10 percent trick, then build from it. All prices are made up.
- A $25 shirt is 40 percent off. What do you pay?
- Your meal is $40. What is a 15 percent tip?
- Sales tax is 7 percent. What is the total on a $20 item?
Check your practice: feedback
- 10 percent of $25 is $2.50. Times 4 is $10 off. You pay $15.
- 10 percent is $4. Half of that, 5 percent, is $2. Add them: $6.
- $20 times 0.07 is $1.40. The total is $21.40.
Common mistakes
- Taking the percent off twice. Find the discount once, then subtract.
- Typing 25 instead of 0.25. Change the percent to a decimal first.
- Forgetting the tax. Add it before you decide you can afford it.
Try it: Practice with real prices
Pick your scenario:
- I eat out sometimes
- I shop sales a lot
- I want to check tax on my receipts
- Find one sale sign or ad.
- Find 10 percent of the price.
- Work out the sale price.
- Find the tax rate on an old receipt.
- Add tax to the sale price.
Do it by: this week
Reflect
Which trick will you use most: sales, tips, or tax?
Check your understanding
1. What is 10 percent of $60?
- $6
- $0.60
- $600
See the answer and feedback
A is correct. Yes. Move the decimal one place left.
B: That is 1 percent. Review The 10 percent trick.
C: That is 10 times bigger. Review The 10 percent trick.
2. A $20 hat is 50 percent off. What do you pay?
- $15
- $10
- $5
See the answer and feedback
B is correct. Yes. Half of $20 is $10.
A: That is 25 percent off. Review Sale prices.
C: That is 75 percent off. Review Sale prices.
3. Tax is 5 percent. The item is $40. What is the tax?
- $20
- $0.20
- $2
See the answer and feedback
C is correct. Yes. 40 times 0.05 is 2.
A: That is half the price. Review Sales tax.
B: Check your decimal. Review Sales tax.
Takeaway: percents
- 10 percent: move the decimal one place left
- Build 20, 15, or 40 percent from 10 percent
- Tax adds to the price. A discount comes off
- Check: does the answer make sense?
Recap
- Percent means out of 100.
- 10 percent: move the decimal one place left.
- Sale price is price minus the discount.
- Tip and tax: price times the percent as a decimal.
You now have: a sale price with tax that you worked out.
Next up
Next you will estimate a total before you pay. Go to Lesson 3: Estimate before you pay.
Sources
- FTC Guides Against Deceptive Pricing, 16 CFR 233.1 Former price comparisons (eCFR) (current as of Oct 29, 2026)
- NIST SP 1181: Unit Pricing Guide, A Best Practice Approach to Unit Pricing (2025 edition) (current as of Oct 29, 2026)
