Percent Proportion

Use the proportion part/whole = percent/100 to find a missing part, whole or percent with cross multiplication.

Intermediate25 min18 problems
Practice Problems

Definition

The percent proportion says that a part compared to its whole is the same as the percent compared to 100:
partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100}
  • The whole is the total amount (it is usually the number after the word of).
  • The part is a piece of that total (it is usually the number next to the word is).
  • pp is the percent, the number in front of the % sign.
In every percent problem, two of these three numbers are known and one is missing. Put the known numbers in the proportion, write a letter for the missing one, then cross multiply and divide.
Example: What percent of 40 is 12?
1240=p100⇒40p=12×100=1200⇒p=30\frac{12}{40} = \frac{p}{100} \quad\Rightarrow\quad 40p = 12 \times 100 = 1200 \quad\Rightarrow\quad p = 30
So 12 is 30%30\% of 40.

Try it now

Which proportion matches the question "What percent of 50 is 20?"

Worked Examples

What percent of 40 is 12?
1

Identify the part and the whole

The whole comes after "of": 40. The part comes with "is": 12. The percent is missing. → part = 12, whole = 40
2

Write the proportion

1240=p100\frac{12}{40} = \frac{p}{100}
3

Cross multiply

40×p=12×10040 \times p = 12 \times 100 → 40p=120040p = 1200
4

Divide by 40

p=1200÷40p = 1200 \div 40 → p=30p = 30

Common Mistakes

Putting the whole on top: writing 4012=p100\frac{40}{12} = \frac{p}{100} for "What percent of 40 is 12?"

Why it's wrong: The fraction on the left must be part over whole. Flipping it gives p≈333p \approx 333, which is impossible here because 12 is smaller than 40.

Correct: Part on top, whole on the bottom: 1240=p100\frac{12}{40} = \frac{p}{100}, so p=30p = 30.

Writing the percent in the part spot: 30x=15100\frac{30}{x} = \frac{15}{100} for "15 is 30% of what number?"

Why it's wrong: The number with the % sign always goes over 100. The other known number is the part or the whole.

Correct: 15x=30100\frac{15}{x} = \frac{30}{100}, so 30x=150030x = 1500 and x=50x = 50.

Multiplying straight across instead of crosswise: 12×p=40×10012 \times p = 40 \times 100

Why it's wrong: Cross multiplication pairs each numerator with the OTHER fraction's denominator.

Correct: Multiply on the diagonals: 12×100=40×p12 \times 100 = 40 \times p.

Answering 0.3 when the question asks for the percent

Why it's wrong: In the proportion pp is already the number of percent (out of 100), not a decimal.

Correct: If p=30p = 30, the answer is 30%30\%. Write the % sign.

Interactive Visual

12 is 30% of 40

Part: 30%
4
4
4
= 12
Rest: 70%
4
4
4
4
4
4
4
= 28
Whole: 100% = 40
= 10 (40)
Part: 30% value:12
Rest: 70% value:28

Equation Solver

Solve for xx:

1240=x100\frac{12}{40} = \frac{x}{100}
1
1240=x100\frac{12}{40} = \frac{x}{100}
Cross-multiply: a⋅d=b⋅ca \cdot d = b \cdot cNumerator of each side times the denominator of the other side. It is the same as multiplying both sides by both denominators.

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.

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Practice Problems

18 problems
Problem 1 of 18
Easy
Which proportion matches the question "What percent of 50 is 20?"

Why It Matters

You already know how to find a percent of a number. Real problems are often the other way around:
  • Test scores: You got 34 out of 40 points. What percent is that?
  • Surveys: 45 students chose basketball, which is 18% of everyone asked. How many students were asked?
  • Shopping: You saved 12 dollars, which was 15% off. What was the original price?
  • Sports: A player made 27 of 30 free throws. What is the success rate?
The percent proportion handles all three kinds of questions with one setup, so you do not need a separate rule for each. In the next lesson you will meet the other method, the percent equation (part = rate × whole), which solves the same problems in one line.

Real World Applications

Test Scores

Teachers turn points into percents so that tests with different numbers of questions can be compared.

Example:

18 points out of 24 is 1824=p100\frac{18}{24} = \frac{p}{100}, so p=75p = 75, a score of 75%75\%.

1Try It Yourself

Liam answered 27 of the 30 questions on a quiz correctly.

What percent did Liam get right?

Step 1: Write the mathematical expression

Solve 2730=p100\frac{27}{30} = \frac{p}{100}: calculate 27×100÷3027 \times 100 \div 30

Surveys and Polls

Polls report results as percents. If you know the percent and how many people gave an answer, you can find how many people were asked.

Example:

If 30 people are 25%25\% of a group, then 30x=25100\frac{30}{x} = \frac{25}{100} and the group has 120 people.

2Try It Yourself

In a town poll, 120 people said yes to a new park. They were 60% of all the people asked.

How many people were asked in the poll?

Step 1: Write the mathematical expression

Solve 120x=60100\frac{120}{x} = \frac{60}{100}: calculate 120×100÷60120 \times 100 \div 60

Shopping and Discounts

A receipt often shows how much you saved but not the original price. The percent proportion finds the original price.

Example:

Saving 6 dollars at 20%20\% off means 6x=20100\frac{6}{x} = \frac{20}{100}, so the original price was 30 dollars.

3Try It Yourself

A jacket is 15% off and you save 12 dollars.

What was the original price of the jacket in dollars?

Step 1: Write the mathematical expression

Solve 12x=15100\frac{12}{x} = \frac{15}{100}: calculate 12×100÷1512 \times 100 \div 15

Key Takeaways

  • 1The percent proportion is partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100}
  • 2The whole usually follows the word "of", the part usually goes with "is"
  • 3The number with the % sign always goes over 100
  • 4Write a letter for the missing number, cross multiply, then divide
  • 5The same setup finds the part, the whole or the percent
  • 6Check your answer: the part should be that percent of the whole

Frequently Asked Questions

Ask: which number is the total? That is the whole (often after "of"). The part is a piece of it (often next to "is"). If the percent is under 100, the part is smaller than the whole.
Ask: which number is the total? That is the whole (often after "of"). The part is a piece of it (often next to "is"). If the percent is under 100, the part is smaller than the whole.
Yes, when the percent is more than 100. For example, 60 is 150%150\% of 40: 6040=p100\frac{60}{40} = \frac{p}{100} gives p=150p = 150.
Two equal fractions stay equal when you multiply both sides by both denominators. For ab=cd\frac{a}{b} = \frac{c}{d} this gives a×d=b×ca \times d = b \times c.
They are two ways to write the same fact. The proportion compares two fractions; the percent equation (next lesson) writes part = rate × whole with the percent as a decimal. Both give the same answer.

Glossary

Percent proportion
The equation partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100} that links a part, a whole and a percent.
Part
The amount that is a portion of the whole, for example the correct answers on a test.
Whole
The total amount, which counts as 100%100\%.
Percent
A number out of 100, written with the % sign.
Cross multiplication
In ab=cd\frac{a}{b} = \frac{c}{d}, multiplying on the diagonals to get a×d=b×ca \times d = b \times c.

Formula Card

Percent proportion

partwhole=p100\frac{\text{part}}{\text{whole}} = \frac{p}{100}

Part over whole equals percent over 100.

Missing percent

p=part×100wholep = \frac{\text{part} \times 100}{\text{whole}}

What percent of 40 is 12? $p = 1200 \div 40 = 30$

Missing whole

whole=part×100p\text{whole} = \frac{\text{part} \times 100}{p}

15 is 30% of what? $1500 \div 30 = 50$

Missing part

part=p×whole100\text{part} = \frac{p \times \text{whole}}{100}

35% of 60: $2100 \div 100 = 21$

Today's challengeOne problem, three tries

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