What you need to know
A decimal point separates whole numbers from parts of a whole. The first place after the point is tenths (one whole split into 10 equal parts) and the second place is hundredths (one whole split into 100 equal parts). So 0.7 is 710 and 0.07 is 7100. They look similar but 0.7 is ten times bigger.
| Decimal | Fraction | In words |
|---|
| 0.3 | 310 | three tenths |
| 0.09 | 9100 | nine hundredths |
| 0.45 | 45100 | forty-five hundredths |
| 2.6 | 2610 | two and six tenths |
Place value chart. Every digit has a place: ones, then the decimal point, then tenths, then hundredths. In 3.47 there are 3 ones, 4 tenths and 7 hundredths. Writing a number as the sum of its places is called expanded form.
Write 3.47 in expanded form
- The 3 is in the ones place: 3.
- The 4 is in the tenths place: 4 tenths = 0.4 = 410.
- The 7 is in the hundredths place: 7 hundredths = 0.07 = 7100.
- So 3.47 = 3 + 0.4 + 0.07.
Tenths and hundredths are linked. 10 hundredths make 1 tenth, so 0.10 is the same as 0.1, and 0.50 = 0.5 = 12. Adding a zero on the end of a decimal does not change its value, which is very handy when you compare decimals.
Which is larger: 0.6 or 0.58?
- Give both numbers the same number of decimal places: 0.6 = 0.60.
- 0.60 is 60 hundredths. 0.58 is 58 hundredths.
- 60 is more than 58, so 0.6 is larger.
- Money check: 60 cents is more than 58 cents.
Decimals in real life. Money is written in hundredths: $2.35 is 2 dollars, 3 ten-cent coins (tenths of a dollar) and 5 cents (hundredths). Lengths work the same way: 1.25 m is 1 metre and 25 centimetres, because a centimetre is one hundredth of a metre.
Order 0.3, 0.25 and 0.09 from smallest to largest
- Write them all as hundredths: 0.3 = 0.30, 0.25, 0.09.
- Compare: 30 hundredths, 25 hundredths, 9 hundredths.
- Smallest to largest: 0.09, 0.25, 0.3.
Common mistake: thinking 0.58 is bigger than 0.6 because 58 is bigger than 6. Decimals are not compared by how many digits they have. Compare place by place from the left, or give both numbers the same number of decimal places first (0.60 and 0.58).
Common mistake: reading 0.07 as seven tenths. The 7 is in the second place after the point, so it is seven hundredths. 0.7 is seven tenths.
Check yourself
- Write 810 as a decimal.
- Write 0.04 as a fraction.
- What is the value of the 5 in 2.65?
- Which is larger: 0.4 or 0.39?
- Write 1.72 in expanded form.
- Order 0.5, 0.15 and 0.51 from smallest to largest.
Answers: 1. 0.8 2. 4100 3. 5 hundredths (0.05) 4. 0.4 (it equals 0.40, and 40 hundredths is more than 39 hundredths) 5. 1 + 0.7 + 0.02 6. 0.15, 0.5, 0.51