Adding & Subtracting Decimals — Grade 8 Maths (CAPS)

The one rule that makes decimal addition and subtraction reliable — line up the decimal commas — plus filling gaps with zeros and money problems, with worked examples, a worksheet with memo and a quiz.

Adding and subtracting decimals is exactly the same as with whole numbers, with one extra rule that fixes almost every mistake learners make.

1The golden rule: line up the commas

Write the numbers under each other so the decimal commas line up vertically, then add or subtract as normal.

Worked ExampleCalculate: 12,7 + 3,45

1
Line up the commas and fill the gap with a zero: 12,70 + 3,45.
12,7 and 12,70 are the same value — the zero just keeps the columns tidy.
2
Add the hundredths: 0 + 5 = 5. Add the tenths: 7 + 4 = 11, write 1 carry 1.
Work right to left, exactly like whole numbers.
3
Add the units with the carry: 2 + 3 + 1 = 6, then the tens: 1.
4
Answer: 16,15.
The comma in the answer sits under the commas above it.

Worked ExampleCalculate: 8,3 − 2,75

1
Line up and fill the gap: 8,30 − 2,75.
Writing 8,3 as 8,30 makes the borrowing clear.
2
Hundredths: 0 − 5 needs borrowing → 10 − 5 = 5.
Borrow 1 tenth from the tenths column — it becomes 10 hundredths.
3
Tenths: 2 − 7 needs borrowing → 12 − 7 = 5.
4
Units: 7 − 2 = 5. Answer: 5,55.

⚠️

Do not line the numbers up by their last digits. Writing 12,7 above 3,45 with the 7 above the 5 gives a badly wrong answer. Line up the commas, not the last digits.

2Estimating to check your answer

Before (or after) a decimal calculation, round each number to the nearest whole number and do it in your head. If your exact answer is far from the estimate, you have made a slip.

Worked ExampleEstimate to check: 12,7 + 3,45 = 16,15

1
Round: 12,7 ≈ 13 and 3,45 ≈ 3.
Round to the nearest whole number.
2
13 + 3 = 16.
3
16,15 is very close to 16, so the answer is sensible.
This catches a misplaced decimal comma instantly.

3Money problems

Worked ExampleSipho buys bread for R18,50 and milk for R24,75. He pays with R50. How much change?

1
Total spent: 18,50 + 24,75 = 43,25.
Line up the commas and add.
2
Change: 50,00 − 43,25.
Write R50 as 50,00 so the columns match.
3
= R6,75.
Answer in rands and cents, with the unit.

Practice exercises

Work each one out, then click to reveal the answer.

📄 Worksheet ✅ Memo 📝 Exam paper 🗝️ Marking guide

1
4,6 + 2,35
Show answer ▾Hide answer ▴
4,60 + 2,35 = 6,95
2
13,08 + 7,9
Show answer ▾Hide answer ▴
13,08 + 7,90 = 20,98
3
9,5 − 3,27
Show answer ▾Hide answer ▴
9,50 − 3,27 = 6,23
4
20 − 6,45
Show answer ▾Hide answer ▴
20,00 − 6,45 = 13,55
5
0,75 + 0,8 + 1,2
Show answer ▾Hide answer ▴
2,75
6
Estimate 19,7 + 4,2 by rounding to whole numbers.
Show answer ▾Hide answer ▴
20 + 4 = about 24 (exact: 23,9)
7
A plank is 2,4 m long. 0,85 m is cut off. How much is left?
Show answer ▾Hide answer ▴
2,40 − 0,85 = 1,55 m
8
Ann buys items for R32,40 and R15,85 and pays with R100. What change does she get?
Show answer ▾Hide answer ▴
Total 48,25; 100,00 − 48,25 = R51,75