Percentages: How to Stop Looking Like an Idiot at Restaurants

The Dinner That Made Me Learn Math

Picture this: I'm at a restaurant with friends. The bill comes: $183.47. We need to split it 4 ways and add an 18% tip.

Everyone pulls out their phones. Not to pay—we're all adults with digital wallets—but to open calculator apps.

Dead silence as we all punch in numbers.

"What's 18% of $183.47?" "How do we split $183.47 by 4?" "Wait, do we tip on pre-tax or post-tax?"

Five college-educated adults, completely stumped by basic percentage math. We spent 5 minutes calculating what should've taken 10 seconds.

That's when I realized: I suck at percentages. And I'm guessing you do too.

So I actually sat down and learned this stuff. Turns out, it's way easier than school made it seem.

What Even Is a Percentage?

Per cent = "per hundred" in Latin. That's it. A percentage is just a fraction with 100 on the bottom.

25% = 25/100 = 0.25 = 1/4

Why percentages matter: They let us compare different things on equal footing.

• Company A grew by 500 users
• Company B grew by 50 users

Which is better? No idea without context. But:

• Company A grew by 5%
• Company B grew by 50%

Now we know Company B is crushing it.

The Four Things You Actually Need to Know

1. Finding X% of a Number (The Tip Calculator)

The formula:

Percentage × Number = Result

But here's the mental math shortcut:

1. Find 10% by moving the decimal one place left
2. Multiply or divide from there

Real example:

Bill: $80
Tip: 20%

Step 1: 10% of $80 = $8 (move decimal)
Step 2: 20% = $8 × 2 = $16

Total: $80 + $16 = $96

That's it. 10% is your anchor for everything.

2. Finding What Percentage X Is of Y

The formula:

(X ÷ Y) × 100 = Percentage

Examples:

You scored 45 out of 60 on a test.
What percentage?

45 ÷ 60 = 0.75
0.75 × 100 = 75%

You got a C. Better luck next time.

Your website had 2,000 visitors last month, 3,000 this month.
Growth percentage?

Increase: 3,000 - 2,000 = 1,000
1,000 ÷ 2,000 = 0.5
0.5 × 100 = 50% increase

Nice work!

3. Percentage Increase (Raises, Growth)

Formula:

((New - Old) ÷ Old) × 100 = % Increase

Example:

Salary: $50,000 → $55,000

Increase: ($55,000 - $50,000) ÷ $50,000 × 100
= $5,000 ÷ $50,000 × 100
= 10% raise

Not bad! But inflation was 4%, so real raise is 6%.

4. Finding the Original Price (Sale Shopping)

When you know the sale price and discount:

Sale Price ÷ (1 - Discount%) = Original Price

Example:

You paid $80 for shoes at 20% off.
What was the original price?

You paid 80% of original (100% - 20%)
$80 ÷ 0.80 = $100 original price

Those shoes were $100. You saved $20. Good deal.

Mental Math Shortcuts (So You Don't Look Dumb)

The 10% Rule (Learn This One Trick)

Every percentage calculation starts with 10%:

10% of 450 = 45 (move decimal left once)
10% of 37 = 3.7
10% of 1,250 = 125

Building Other Percentages from 10%

5% = Half of 10%
20% = Double 10%
25% = Quarter of number (or half of half)
30% = 10% × 3
50% = Half of number
75% = 50% + 25%

Quick Reference Table

| Percentage | How to Calculate | Example (of 200) | |------------|------------------|------------------| | 1% | ÷ 100 | 2 | | 5% | ÷ 20 or 10% ÷ 2 | 10 | | 10% | ÷ 10 | 20 | | 15% | 10% + 5% | 30 | | 20% | ÷ 5 or 10% × 2 | 40 | | 25% | ÷ 4 | 50 | | 30% | 10% × 3 | 60 | | 50% | ÷ 2 | 100 | | 75% | 50% + 25% | 150 | | 100% | The number | 200 |

Real-World Applications (Where You'll Actually Use This)

Restaurant Tips (The Classic)

Bill: $47.50
Standard tip: 15-20%

Quick calculation:
10% = $4.75
20% = $9.50 (just double it)
15% = $4.75 + $2.38 = ~$7.13

Tip $7-9 depending on service.

Pro tip: For 18%, do 20% and subtract a bit. Close enough for restaurant math.

Shopping Discounts

Price: $250
Discount: 30% off

Method 1: Calculate discount
30% of $250 = $75
Sale price: $250 - $75 = $175

Method 2: Calculate what you'll pay
You'll pay 70% (100% - 30%)
70% of $250 = $175

Same result, less math.

Understanding Sales Tax

Purchase: $100
Sales tax: 8.5%

Calculation:
8.5% = 10% - 1.5%
10% = $10
1% = $1
1.5% = $1.50

Tax: $10 - $1.50 = $8.50
Total: $108.50

Credit Card Interest (Scary Math)

Balance: $5,000
APR: 19.99%

Monthly interest rate: 19.99% ÷ 12 = 1.67%
Monthly interest: $5,000 × 0.0167 = $83.50

That $83.50 is JUST INTEREST.
None of it pays down your balance.
This is why credit card debt is a trap.

Investment Returns

Investment: $10,000
Annual return: 7%

Year 1: $10,000 × 1.07 = $10,700
Year 2: $10,700 × 1.07 = $11,449
Year 3: $11,449 × 1.07 = $12,250

Rule of 72: 72 ÷ 7 = ~10 years to double

In 10 years: ~$20,000
In 20 years: ~$40,000
In 30 years: ~$80,000

This is why you start investing early.

Business Metrics

Profit Margin:

Revenue: $100,000
Costs: $70,000
Profit: $30,000

Profit margin: $30,000 ÷ $100,000 = 30%

Conversion Rate:

Website visitors: 10,000
Purchases: 150

Conversion rate: 150 ÷ 10,000 = 1.5%

Marketing ROI:

Ad spend: $5,000
Revenue: $20,000

ROI: ($20,000 - $5,000) ÷ $5,000 × 100
= $15,000 ÷ $5,000 × 100
= 300% ROI

Every dollar spent made $3 back.

Common Mistakes (Don't Do These)

Mistake 1: Stacking Discounts Wrong

20% off + 10% off additional

Wrong: 20% + 10% = 30% off
Right: Calculate sequentially

Original: $100
After 20%: $100 × 0.80 = $80
After 10%: $80 × 0.90 = $72

You paid $72, not $70.
Effective discount: 28%, not 30%.

Mistake 2: "Buy One Get One" Confusion

BOGO 50% off, items cost $40 each

You pay: $40 + ($40 × 0.50) = $60
For 2 items worth $80

Actual discount: $20 ÷ $80 = 25%

Not 50% like the sign implies!

Mistake 3: Compounding Confusion

Investment: $10,000
Year 1: +50%
Year 2: -50%

Year 1: $10,000 × 1.50 = $15,000
Year 2: $15,000 × 0.50 = $7,500

Result: -25% (not 0%)

Equal gains and losses don't cancel out.

Quick Formulas Reference

| Operation | Formula | Example | |-----------|---------|---------| | Find X% of Y | (X ÷ 100) × Y | 20% of 50 = 10 | | Find what % X is of Y | (X ÷ Y) × 100 | 10 is 20% of 50 | | % Increase | ((New - Old) ÷ Old) × 100 | 50→60 = 20% increase | | % Decrease | ((Old - New) ÷ Old) × 100 | 60→50 = 16.7% decrease | | Find original after discount | Sale ÷ (1 - Discount%) | $80 at 20% off = $100 original |

Practice Problems (Test Yourself)

Level 1: Easy

1. What is 15% of 200?
2. 45 is what percentage of 180?
3. Increase 80 by 25%

Level 2: Medium

4. $120 item at 30% off. Sale price?
5. Salary $50k → $58k. Percentage increase?
6. $10k investment → $13k over 3 years. Annual return?

Level 3: Tricky

7. 20% off coupon + 30% off sale. Total discount?
8. Lose 20% of portfolio. What % gain to break even?
9. Product costs $100 wholesale. Want 40% profit margin. Retail price?

Answers

1. 30
2. 25%
3. 100
4. $84
5. 16%
6. ~9.14% annually
7. 44% effective (not 50%)
8. 25% (not 20%)
9. $166.67 (not $140)

FAQ: Questions I Actually Had

Q: Fastest way to calculate a 15% tip? A: Find 10% (move decimal), add half of that. $60 bill: $6 + $3 = $9.

Q: Why doesn't 20% off + 20% off = 40% off? A: Second discount applies to reduced price. 20% + (20% of 80%) = 36%.

Q: How do I calculate percentage increase from zero? A: You can't. Use absolute numbers instead.

Q: Difference between markup and margin? A: Markup is % of cost. Margin is % of selling price. They're different!

Q: How do I convert fractions to percentages? A: Divide numerator by denominator, multiply by 100. 3/4 = 75%.

Q: What's a "percentage point"? A: Simple difference. 5% to 7% = 2 percentage points. Not the same as % change.

When to Use a Calculator

Mental math is fine for:

• Restaurant tips (approximate is okay)
• Quick shopping estimates
• Comparing simple percentages

Use a calculator for:

• Financial decisions (investments, loans)
• Business calculations (exact margins)
• Compound interest
• When precision matters

Try our Percentage Calculator when you need to be precise.

The Bottom Line

Percentages aren't hard. We just weren't taught practical shortcuts.

Remember:

1. 10% is your anchor - Everything builds from here
2. Mental math for estimates - Calculators for precision
3. Watch for common traps - Compounding, stacking discounts
4. Practice with real money - Nothing motivates like your own finances

Next time you're at a restaurant, try the mental math. You might surprise yourself.

Want exact calculations? Use our Percentage Calculator with step-by-step solutions.

Stop reaching for your phone. Do the math. Impress your friends.

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Written by someone who finally learned this at age 32