Percentages — ACT Math Guide
Percentages ACT questions appear frequently throughout the math section and test your ability to convert between fractions, decimals, and percentages. These problems involve finding what percent one number is of another, calculating percentage increases and decreases, and solving real-world applications. You'll typically see 3-4 percentage questions among the 60 questions in 60 minutes on the ACT math section. With solid fundamentals and efficient techniques, you can tackle these confidently and boost your ACT math score.
What You Need to Know
Percent means "out of 100" — 25% = 25/100 = 0.25
To find a percentage of a number: multiply the decimal form by the number
To find what percent A is of B: divide A by B, then multiply by 100
Percentage increase: (New - Old)/Old × 100
Percentage decrease: (Old - New)/Old × 100
The whole always equals 100%
📐 KEY FORMULA: Percent = (Part/Whole) × 100
⏱️ ACT TIME TIP: Convert percentages to decimals immediately — it's faster than working with fractions during the 1-minute-per-question pace.
How to Solve Percentages on the ACT
Example Question 1 — Easy/Medium Difficulty
A store marks up the price of a $40 item by 25%. What is the new price?
A) $10
B) $35
C) $50
D) $55
E) $65
Solution:
Step 1: Convert 25% to decimal: 25% = 0.25
Step 2: Find the markup amount: $40 × 0.25 = $10
Step 3: Add markup to original price: $40 + $10 = $50
✅Answer: C — The new price is the original price plus the 25% markup.
Example Question 2 — Hard Difficulty
If 18 is 30% of x, and y is 45% of x, what is the value of y?
A) 24
B) 27
C) 30
D) 36
E) 54
Solution:
Step 1: Find x using "18 is 30% of x": 18 = 0.30x, so x = 18/0.30 = 60
Step 2: Find y using "y is 45% of x": y = 0.45 × 60
Step 3: Calculate: y = 27
✅Answer: B — First solve for x, then use that value to find y.
Common ACT Math Mistakes to Avoid
❌Mistake: Forgetting to convert percentages to decimals before calculating
✅Fix: Always convert first — 15% becomes 0.15 immediately
❌Mistake: Confusing "A is what percent of B" with "what is A percent of B"
✅Fix: "A is what percent of B" means (A/B) × 100
❌Mistake: Adding the percentage directly instead of adding the increase
✅Fix: For 20% increase on $50, add $10 (not $20) to get $60
❌Mistake: Using the wrong base for percentage change problems
✅Fix: Always use the original value as the denominator for percent change
Practice Question — Try It Yourself
A shirt originally costs $80. During a sale, it's marked down 35%. What is the sale price?
A) $28
B) $45
C) $52
D) $68
E) $115
Show Answer
Answer: C — Calculate 35% of $80 = $28, then subtract from original: $80 - $28 = $52
Key Takeaways for the ACT
Convert percentages to decimals immediately for faster calculations
Remember that percentage problems often involve three values: part, whole, and percent
Use your calculator efficiently — it's allowed throughout the entire ACT math section
Watch for percentage change problems that require finding the original value first
Double-check whether you need to add or subtract the calculated percentage
Related ACT Math Topics
Strengthen your ACT math prep with these related topics:
Ratios and proportions →
Basic statistics →