More numbers to 100
Numbers in words, rounding and regrouping
Sometimes numbers are written in words. In this unit you write 2-digit numbers in words, round them to the nearest 10, and regroup them into tens and ones in different ways.
Numbers in words
To write a 2-digit number in words, split it into its tens and its ones. For example, 84 is 80 and 4, which we write as eighty-four. 57 is 50 and 7, which is fifty-seven. We join the tens word and the ones word with a small line called a hyphen, like in thirty-one.
Rounding to the nearest 10
To round a number to the nearest 10, find the two tens it sits between, then choose the closer one. For 74, it sits between 70 and 80. 74 is closer to 70, so it rounds to 70. A quick way is to look at the ones digit. If it is less than 5, round down. If it is 5 or more, round up. So 27 rounds up to 30, and 23 rounds down to 20.
Regrouping tens and ones
You can regroup a number into tens and ones in different ways, and it still has the same value. 31 is normally 3 tens and 1 one. If you swap 1 ten for 10 ones, you get 2 tens and 11 ones (20 + 11 = 31). You can also write it as a sum: 58 = 50 + 8 = 40 + 18 = 30 + 28. Each way makes the same number.
Worked examples
Write 57 in words
- Split 57 into tens and ones: 50 and 7.
- Write the tens word: fifty. Write the ones word: seven.
- Join them with a hyphen.
Answer: 57 in words is fifty-seven
Round 74 to the nearest 10
- 74 sits between 70 and 80.
- Look at the ones digit: 4. It is less than 5.
- So round down to 70.
Answer: 74 rounds to 70
Regroup 31
- Start with the normal grouping: 3 tens and 1 one.
- Swap 1 ten for 10 ones.
- Now you have 2 tens and 11 ones, which is still 31.
Answer: 31 = 2 tens and 11 ones
Where you see this in real life
- Reading a number written in words, like on a card or a label.
- Rounding a number to the nearest 10 to make a quick estimate.
- Swapping 1 ten for 10 ones, like changing a ten note for ten coins.
Key words
- Hyphen
- The small line that joins the two parts of a number word, like in thirty-one.
- Round
- To change a number to the nearest 10, so it is easier to work with.
- Nearest 10
- The ten that a number is closest to, such as 70 for 74.
- Regroup
- To split a number into tens and ones in a different way, keeping the same value.
- Tens and ones
- The two parts of a 2-digit number: the tens on the left and the ones on the right.