Stage 8 · Fractions & decimals
Adding and subtracting decimals
Match the places, work as whole numbers.
How to do it
Method
Match the places, work as whole numbers
3.6 + 2.7: 36 + 27 = 63 → 6.3
Another way
Columns, points lined up
Tenths 6 + 7 = 13, write 3, carry 1; ones 3 + 2 + 1 = 6 → 6.3
In full
Give both numbers the same number of decimal places, work them as whole numbers, then put the point back with that many places: 3.6 + 2.7 → 36 + 27 = 63 → 6.3; 0.75 + 0.5 → 0.75 + 0.50 → 75 + 50 = 125 → 1.25. A whole number can be written as a decimal too: 5 − 2.7 → 5.0 − 2.7 → 50 − 27 = 23 → 2.3. Zeros at the end of a decimal drop off: 6.0 = 6. Or work in columns with the points lined up — the same idea.
Worked examples
Have a go first, then open each one for the answer and the working.
Example 1
9.1 + 3.3
Show the answer and working
Answer: 9.1 + 3.3 = 12.4
9.1 + 3.3: both have one place, so work as whole numbers, 91 + 33 = 124, then put the point back one place → 12.4
Example 2
0.9 − 0.27
Show the answer and working
Answer: 0.9 − 0.27 = 0.63
0.9 − 0.27: write 0.9 as 0.90, both have two places, so work as whole numbers, 90 − 27 = 63, then put the point back two places → 0.63
Example 3
0.63 − 0.5
Show the answer and working
Answer: 0.63 − 0.5 = 0.13
0.63 − 0.5: write 0.5 as 0.50, both have two places, so work as whole numbers, 63 − 50 = 13, then put the point back two places → 0.13
Example 4
0.82 − 0.6
Show the answer and working
Answer: 0.82 − 0.6 = 0.22
0.82 − 0.6: write 0.6 as 0.60, both have two places, so work as whole numbers, 82 − 60 = 22, then put the point back two places → 0.22
Practise adding and subtracting decimals in the game →Stage 8 · Fractions & decimals · Free to start, unlimited questions, no download