Adding and Subtracting Decimals

The Golden Rule

Always line up the decimal points!

This is the most important thing to remember when adding or subtracting decimals.

Adding Decimals

Example: $3.45 + 12.8$

| Step | What to do | |------|-----------| | 1. Line up decimals | Stack the numbers with decimal points aligned | | 2. Add placeholder zeros | Write $12.8$ as $12.80$ | | 3. Add column by column | Start from the right | | Answer | $3.45 + 12.80 = 16.25$ |

$3.45 + 12.8 = 16.25$

Tip: Add zeros as placeholders so both numbers have the same number of decimal places!

Subtracting Decimals

Example: $15.6 - 7.38$

| Step | What to do | |------|-----------| | 1. Line up decimals | Stack the numbers with decimal points aligned | | 2. Add placeholder zeros | Write $15.6$ as $15.60$ | | 3. Subtract column by column | Borrow when needed | | Answer | $15.60 - 7.38 = 8.22$ |

$15.6 - 7.38 = 8.22$

Money Problems

Decimals are used with money all the time!

Example: You have \20.00$and spend$$12.75$. How much is left?

$20.00 - 12.75 = 7.25$

You have \7.25$ left.

Example: You buy items for \3.49$,$$5.25$, and$$1.99$. What's the total?

$3.49 + 5.25 + 1.99 = 10.73$

Total: \10.73$

Word Problem Strategy

1. Read carefully — decide whether to add or subtract
2. Line up the decimal points — always!
3. Add placeholder zeros if the numbers have different decimal places
4. Check your answer — does it make sense?
5. Label your answer — include units (dollars, meters, etc.)

Practice Tips

• Estimation helps: Round to whole numbers first to check if your answer is reasonable
• Common mistake: Forgetting to line up the decimals — this leads to wrong answers every time!
• Money is great practice: Use receipts and price tags to practice adding decimals