Add and subtract decimals
Adding and Subtracting Decimals teaches the vertical alignment method that prevents place-value errors. From OpenStax Prealgebra 2E, three steps: write numbers with decimal points aligned vertically, use zeros as placeholders where needed, then add or subtract as whole numbers and place the decimal directly below the other decimal points. To add 18.5 + 3.45, rewrite as 18.50 + 3.45 = 21.95. To subtract 50 − 22.18, treat 50 as 50.00 and compute 50.00 − 22.18 = 27.82. The decimal point in the answer aligns with the decimal points in the problem.
Key Concepts
Property How to add or subtract decimals: Step 1. Write the numbers vertically so the decimal points line up. Step 2. Use zeros as place holders, as needed. Step 3. Add or subtract the numbers as if they were whole numbers. Then place the decimal in the answer under the decimal points in the given numbers.
Examples To add $18.5 + 3.45$, we write the numbers vertically, aligning the decimal points and adding a placeholder zero: $18.50 + 3.45 = 21.95$.
To subtract $50 22.18$, we treat $50$ as $50.00$ to align the decimal points and use placeholders: $50.00 22.18 = 27.82$.
Common Questions
How do you add or subtract decimals?
Write the numbers vertically with decimal points aligned, use zeros as placeholders, then add or subtract and place the decimal directly below.
What is 18.5 + 3.45?
Rewrite as 18.50 + 3.45. Add: 18.50 + 3.45 = 21.95.
What is 50 − 22.18?
Treat 50 as 50.00. Subtract: 50.00 − 22.18 = 27.82.
Why do you align decimal points?
Aligning decimal points ensures you add or subtract digits in the same place value — tenths with tenths, hundredths with hundredths.
What are placeholder zeros?
Zeros added to the right of a decimal number to match decimal places in the other number. 18.5 becomes 18.50 so both numbers have hundredths.
Where does the decimal point go in the answer?
Directly below the decimal points in the numbers being added or subtracted — they all line up in the same column.