8.8 + 2.1 = 10.9. 8.8 + 2.1: both have one place, so work as whole numbers, 88 + 21 = 109, then put the point back one place → 10.9

0.78 − 0.31 = 0.47. 0.78 − 0.31: both have two places, so work as whole numbers, 78 − 31 = 47, then put the point back two places → 0.47

3.9 − 1.9 = 2. 3.9 − 1.9: both have one place, so work as whole numbers, 39 − 19 = 20, then put the point back one place → 2.0 = 2

Decimals · Usually met around P4

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 out 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 can be dropped: 6.0 = 6. Or work in columns with the decimal points lined up. It's the same idea.

Worked examples

Have a go first, then open each one for the answer and the working.

Example 1
8.8 + 2.1
Show the answer and working
Answer: 8.8 + 2.1 = 10.9
8.8 + 2.1: both have one place, so work as whole numbers, 88 + 21 = 109, then put the point back one place → 10.9
Example 2
0.78 − 0.31
Show the answer and working
Answer: 0.78 − 0.31 = 0.47
0.78 − 0.31: both have two places, so work as whole numbers, 78 − 31 = 47, then put the point back two places → 0.47
Example 3
3.9 − 1.9
Show the answer and working
Answer: 3.9 − 1.9 = 2
3.9 − 1.9: both have one place, so work as whole numbers, 39 − 19 = 20, then put the point back one place → 2.0 = 2

Practise it in the game →Decimals · Free to start, unlimited questions, no download