Decimals and Rounding — ACT Math Guide
Decimals and rounding ACT questions test your ability to work with non-whole numbers and express answers to specific decimal places. These problems involve converting between fractions and decimals, performing operations with decimal numbers, and rounding results to the nearest tenth, hundredth, or whole number. Pre-algebra topics like decimals appear in roughly 8-10 questions out of the 60 questions in 60 minutes on the ACT math section. With solid fundamentals and quick calculation skills, you can tackle these problems confidently and boost your ACT math score.
What You Need to Know
Place value: tenths (0.1), hundredths (0.01), thousandths (0.001)
Converting fractions to decimals by dividing numerator by denominator
Adding, subtracting, multiplying, and dividing decimal numbers
Rounding rules: 5 or higher rounds up, 4 or lower rounds down
Estimating decimal calculations for quick answer elimination
📐 KEY FORMULA: To round to n decimal places, look at the (n+1)th decimal place
⏱️ ACT TIME TIP: Use your calculator for decimal operations — it's allowed throughout the entire ACT Math section and saves precious time
How to Solve Decimals and Rounding on the ACT
Example Question 1 — Easy/Medium Difficulty
What is 3.847 rounded to the nearest hundredth?
A) 3.8
B) 3.84
C) 3.85
D) 3.9
E) 4.0
Solution:
Step 1: Identify the hundredths place (second digit after decimal point): 3.847
Step 2: Look at the next digit (thousandths place): 7
Step 3: Since 7 ≥ 5, round up the hundredths digit from 4 to 5
✅Answer: C — The hundredths place rounds up because the next digit is 7
Example Question 2 — Hard Difficulty
A rectangular garden measures 12.375 feet by 8.64 feet. What is the area of the garden rounded to the nearest tenth of a square foot?
A) 106.9 square feet
B) 107.0 square feet
C) 107.1 square feet
D) 107.2 square feet
E) 108.0 square feet
Solution:
Step 1: Calculate the area: 12.375 × 8.64 = 106.92 square feet
Step 2: Identify the tenths place: 106.92
Step 3: Look at the hundredths place: 2
Step 4: Since 2 < 5, round down to 106.9
✅Answer: A — The area rounds down to 106.9 because the hundredths digit is 2
Common ACT Math Mistakes to Avoid
❌Mistake: Forgetting which direction to round when the digit is exactly 5
✅Fix: Always round up when the digit is 5 or higher
❌Mistake: Rounding to the wrong place value (tenths vs hundredths)
✅Fix: Circle the target place value before solving
❌Mistake: Making arithmetic errors with decimal calculations
✅Fix: Use your calculator and double-check decimal point placement
❌Mistake: Converting fractions incorrectly to decimals
✅Fix: Remember that 1/4 = 0.25, 1/3 = 0.333..., 1/8 = 0.125
Practice Question — Try It Yourself
The expression 4.73 × 2.1 ÷ 0.6 equals approximately which value when rounded to the nearest whole number?
A) 16
B) 17
C) 18
D) 19
E) 20
Show Answer
Answer: B — Calculate: 4.73 × 2.1 ÷ 0.6 = 9.933 ÷ 0.6 = 16.555, which rounds to 17
Key Takeaways for the ACT
Master the rounding rule: 5 and above rounds up, 4 and below rounds down
Always identify the target place value before rounding
Use your calculator efficiently for decimal operations on the ACT math section
Practice converting common fractions to decimals for faster recognition
Check your decimal point placement in calculations
Related ACT Math Topics
Strengthen your ACT math prep with these related topics:
Fractions and mixed numbers →
Percents and percent change →