Decimals plus and minus
You learn to add and subtract decimals by thinking of them as hundredths.
How to work it out
- Write numbers as hundredths: 0.25 is 25 hundredths.
- Calculate with integers.
- Convert back: 75 hundredths is 0.75.
A worked example
0.25 + 0.5
- Write them as hundredths: 25 and 50.
- Work it out: 25 + 50 = 75.
- Put it back: 75 hundredths is 0.75.
More examples
1 − 0.55
- Write them as hundredths: 100 and 55.
- Work it out: 100 − 55 = 45.
- Put it back: 45 hundredths is 0.45.
1.5 + 2.5
- Write them as hundredths: 150 and 250.
- Work it out: 150 + 250 = 400.
- Put it back: 400 hundredths is 4.
Try it yourself
0.25 + 0.5 =
2.5 − 1.25 =
Watch out
You count 0.5 as 5 hundredths: 0.25 + 0.5 becomes 0.30.
0.5 is 5 tenths, and that is 50 hundredths. Fill in a 0: 0.50.
You work out the part after the decimal point on its own: 1.5 + 2.5 becomes 3.10.
Ten tenths make one whole. Work in hundredths and it comes out right: 150 + 250 = 400.
Tips
- Write 0.5 as 0.50 and 1 as 1.00, then you see the hundredths.
Why it works
You can already work with whole numbers. 0.25 is 25 hundredths, so 0.25 + 0.50 is just 25 + 50 hundredths. At the end you put the decimal point back.