Three-digit Numbers

1Summary 重點摘要

Know hundreds, tens, units place, learn to read and write 3-digit numbers.

2Vocabulary cards Vocabulary

3Step by step Steps

1. Step 1: Know three placesA three-digit number is made of three digits, and each digit sits in its own place, called a place value. From the right, the first place is the ones, the second is the tens, and the third is the hundreds. When writing a number, each place holds only one digit and the order must not be mixed up: 237 and 327 use the same digits, but because the places differ, they are two different numbers. A common mistake is to squeeze digits together or leave a place empty, writing three hundred and five as 35 and dropping the middle 0; the correct way is to let every digit, including 0, occupy its own place with no gap left over.
2. Step 2: How much each placeThe ones place tells how many ones, the tens place how many tens, and the hundreds place how many hundreds. The same digit means different amounts in different places: 5 in the ones place is 5, in the tens place it is 50, and in the hundreds place it is 500. For example, 452 has 4 hundreds, 5 tens and 2 ones. A common mistake is to compare digits one by one and think 396 is bigger than 401 because 9 is bigger than 0; in fact we look at the hundreds place first: 4 is bigger than 3, so 401 is greater than 396. The higher the place, the greater its effect on size, so always start comparing from the hundreds.
3. Step 3: Example: 123Let us try 123: the hundreds digit is 1, so 1 hundred is 100; the tens digit is 2, so 2 tens is 20; the ones digit is 3, so 3 ones is 3. Add the three parts: 100 + 20 + 3 = 123. This method is called splitting by place value. Try again with 608: the hundreds digit is 6, which is 600; the tens digit is 0, so there are no tens; the ones digit is 8. A common mistake is to write 608 as 68, forgetting that 0 still occupies a place. Keep each place separate, add them in order, and check by adding the three parts back once more.
4. Step 4: Read the numberRead a three-digit number from the highest place: hundreds first, then tens, then ones. For example, 123 is read as 'one hundred and twenty-three' and 725 as 'seven hundred and twenty-five'. Reading place by place, in order, keeps us from reading it wrongly. A common mistake is to read from the ones backwards, saying 123 as 'three, two, one', or to skip a place, reading 340 as 'thirty-four'. Before reading, find where the hundreds place is, then read each place from left to right; there is no need to be fast, accuracy matters most.
5. Step 5: Zero in the middleSome three-digit numbers have a 0 in the middle, such as 105 and 308. When reading, the middle 0 must be said as 'zero', so 105 is read as 'one hundred and five' and 308 as 'three hundred and eight'. If the zero is skipped, someone may hear 15 or 38, which means something completely different. A common mistake is to think the 0 need not be written or read, saying 105 as 'one hundred five', which does not make the empty tens place explicit. The right way is to read the middle zero clearly, so the listener knows exactly that the tens place holds nothing, and a dictation will not go wrong.
6. Step 6: Zero at the endSome three-digit numbers end with 0, such as 120 and 200. When reading, the zero at the end is not said: 120 is read as 'one hundred and twenty', and 200 as 'two hundred'. But when writing, the 0 at the end must not be dropped, or 120 would become 12, ten times smaller. A common mistake is to hear 'one hundred and twenty' in a dictation and write only 12, or to forget the trailing zero in a calculation, working out 120 + 30 as 15. Remember: the final zero may be left out when reading, but not one 0 may be missing when writing; the two rules are not the same.

4Diagrams Diagrams

5Worked example Example

6Common mistakes Mistakes

7Quick check Quick check

8Recap Recap

• 3-digit has hundreds, tens, units
• Count from right: units, tens, hundreds
• Zero in middle must read as 'zero'
• Zero at end don't read
• Each place represents how many hundreds, tens, ones