Percent Equation

Solve any percent problem with one equation: part = rate × whole. Find the part, the whole or the percent, even above 100% and below 1%.

Intermediate25 min18 problems
Practice Problems

Definition

The percent equation connects three quantities with one multiplication:
part=rate×whole\text{part} = \text{rate} \times \text{whole}
  • The whole is the total you take a percent of (100%).
  • The rate is the percent, written as a decimal: 25%=0.2525\% = 0.25, 150%=1.5150\% = 1.5, 0.4%=0.0040.4\% = 0.004.
  • The part is the amount that the percent describes.
Translating words into the equation:
  • "is" becomes ==
  • "of" becomes ×\times
  • "what number" or "what percent" becomes the unknown xx
So "30 is 25% of what number?" becomes 30=0.25x30 = 0.25x.
If you know two of the three quantities, you can always find the third:
  • part: multiply, part=rate×whole\text{part} = \text{rate} \times \text{whole}
  • whole: divide, whole=part÷rate\text{whole} = \text{part} \div \text{rate}
  • rate: divide, rate=part÷whole\text{rate} = \text{part} \div \text{whole}, then multiply by 100 to get the percent

Try it now

Which equation matches the question "What number is 40% of 65?"

Worked Examples

What number is 35% of 80?
1

Translate the words

"What number" is xx, "is" is ==, "of" is ×\times → x=35%×80x = 35\% \times 80
2

Write the rate as a decimal

35%=35÷100=0.3535\% = 35 \div 100 = 0.35 → x=0.35×80x = 0.35 \times 80
3

Multiply

0.35×80=280.35 \times 80 = 28 → x=28x = 28

Common Mistakes

Using the percent number instead of the decimal: writing 30=25x30 = 25x and getting x=1.2x = 1.2

Why it's wrong: The rate in the equation must be a decimal. 25% means 25 per hundred, which is 0.250.25, not 2525.

Correct: Divide the percent by 100 first: 30=0.25x30 = 0.25x, so x=120x = 120.

Mixing up the part and the whole: for "30 is 25% of what number?" calculating 0.25×30=7.50.25 \times 30 = 7.5

Why it's wrong: The number after "of" is the whole. Here the whole is the unknown, and 30 is the part, so 30 goes on the left of the equal sign.

Correct: Translate word by word: 30=0.25×x30 = 0.25 \times x, so x=120x = 120.

Forgetting to multiply by 100 when finding the rate: answering "0.4%" for "18 is what percent of 45?"

Why it's wrong: Dividing part by whole gives the rate as a decimal (0.40.4). A percent is that decimal times 100.

Correct: 18÷45=0.418 \div 45 = 0.4, and 0.4×100=400.4 \times 100 = 40, so the answer is 40%40\%.

Writing a small percent with the wrong decimal: 0.4%=0.040.4\% = 0.04 or 0.4%=0.40.4\% = 0.4

Why it's wrong: Every percent is divided by 100, also small ones. Move the decimal point two places to the left: 0.4→0.0040.4 \to 0.004.

Correct: 0.4%=0.0040.4\% = 0.004, so 0.4% of 500 is 0.004×500=20.004 \times 500 = 2.

Interactive Visual

Equation Solver

Solve for xx:

0.25x=300.25x = 30
1
0.25x=300.25x = 30
Multiply both sides by the LCD, 44Now there are no fractions left

Tips:

  • • Whatever you do to one side, do to the other
  • • Undo operations in reverse order (PEMDAS backwards)
  • • Check your answer by substituting back

Follow along as we solve the equation step by step.

Ratio Tape Diagram

25% (the part)
30
= 30
75% (the rest)
30
30
30
= 90
100% = the whole
= 4 (120)
If total is:
25% (the part) value:30
75% (the rest) value:90

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Practice Problems

18 problems
Problem 1 of 18
Easy
Which equation matches the question "What number is 40% of 65?"

Why It Matters

In the lesson "Finding Percent of a Number" you always knew the whole and the percent. Real problems often hide a different unknown:
  • Shopping: you paid a 45 dollar deposit, which is 15% of the price. What is the full price? (the whole is missing)
  • Grades: you got 34 of 40 points. What percent is that? (the rate is missing)
  • Savings: an account pays 0.8% interest per year. How much must you save to earn 36 dollars? (a rate below 1%)
  • Growth: a city now has 260% of its old population. (a rate above 100%)
The percent equation handles all of these with the same short line, part=rate×whole\text{part} = \text{rate} \times \text{whole}, and the equation solving you already know.

Real World Applications

Paying a Deposit

Stores often ask for a deposit that is a percent of the full price. The deposit is the part, and the full price is the whole you may need to find.

Example:

A 60 dollar deposit that is 20% of the price means 60=0.2x60 = 0.2x, so the price is 60÷0.2=30060 \div 0.2 = 300 dollars.

1Try It Yourself

Mia pays a deposit of 45 dollars for a bike. The deposit is 15% of the full price.

What is the full price of the bike in dollars?

Step 1: Write the mathematical expression

Divide the part by the rate:

Test Scores and Free Throws

Teachers and coaches turn "how many out of how many" into a percent. Here the rate is the unknown.

Example:

A basketball player makes 34 of 40 free throws: 34=r×4034 = r \times 40, r=0.85r = 0.85, so she makes 85%85\% of her shots.

2Try It Yourself

On a test worth 80 points, Leo scores 68 points.

What percent of the points did Leo get? Give the number without the % sign.

Step 1: Write the mathematical expression

Divide the part by the whole and multiply by 100:

Savings Interest Below 1%

Savings accounts often pay less than 1% interest per year. The interest is the part, the money in the account is the whole.

Example:

At 0.5% interest, 2400 dollars earns 0.005×2400=120.005 \times 2400 = 12 dollars in one year.

3Try It Yourself

A savings account pays 0.8% interest per year. After one year Sara earned 36 dollars of interest.

How many dollars were in Sara's account?

Step 1: Write the mathematical expression

Divide the interest by the rate as a decimal:

Key Takeaways

  • 1The percent equation is part=rate×whole\text{part} = \text{rate} \times \text{whole}, with the rate written as a decimal
  • 2Translate word by word: "is" becomes ==, "of" becomes ×\times, "what" becomes the unknown
  • 3To find the whole, divide: whole=part÷rate\text{whole} = \text{part} \div \text{rate}
  • 4To find the percent, divide part by whole and multiply by 100
  • 5Above 100% the rate is greater than 1 (150%=1.5150\% = 1.5) and the part is bigger than the whole; below 1% the rate starts with 0.000.00 (0.4%=0.0040.4\% = 0.004)
  • 6The equation and the proportion partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100} always give the same answer

Frequently Asked Questions

Both always give the same answer. The equation part=rate×whole\text{part} = \text{rate} \times \text{whole} is shorter when you can turn the percent into a decimal easily. The proportion partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100} (lesson "Percent Proportion") keeps the percent as a whole number and can feel safer with tricky decimals. Pick the one you find clearer.
Both always give the same answer. The equation part=rate×whole\text{part} = \text{rate} \times \text{whole} is shorter when you can turn the percent into a decimal easily. The proportion partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100} (lesson "Percent Proportion") keeps the percent as a whole number and can feel safer with tricky decimals. Pick the one you find clearer.
Yes, when the percent is above 100%. For example, 150% of 28 is 42, which is bigger than 28. In growth, prices and populations this happens often.
Look for the word "of": the whole usually comes right after it. In "18 is what percent of 45", the whole is 45. The number next to "is" is usually the part.
Part divided by whole gives the rate as a decimal, for example 0.40.4. A percent means "per hundred", so 0.4=40100=40%0.4 = \frac{40}{100} = 40\%.

Glossary

Percent equation
The equation part=rate×whole\text{part} = \text{rate} \times \text{whole} that links a percent with two amounts.
Whole
The total amount you take a percent of; it stands for 100%. Usually it comes after the word "of".
Part
The amount that a percent of the whole is equal to.
Rate
The percent written as a decimal, for example 35%=0.3535\% = 0.35.
Unknown
The missing quantity in the equation, written with a letter such as xx or rr.

Formula Card

Percent equation

part=rate×whole\text{part} = \text{rate} \times \text{whole}

Use it to find the part: multiply the rate (as a decimal) by the whole.

Finding the whole

whole=part÷rate\text{whole} = \text{part} \div \text{rate}

Divide the part by the rate as a decimal, for example $30 \div 0.25 = 120$.

Finding the percent

rate=part÷whole\text{rate} = \text{part} \div \text{whole}

Divide the part by the whole, then multiply by 100 to get the percent.

Today's challengeOne problem, three tries

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Percent Proportion

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