Addition, subtraction & money
Addition
In this unit you add numbers with up to three digits. You learn what to do when you have too many ones for the ones place, and how to estimate first by rounding.
Line up the places
Write the numbers so the hundreds, tens and ones line up in columns. Always start adding from the ones (the right-hand column), then the tens, then the hundreds.
Regrouping the ones
If the ones add up to 10 or more, you have too many for the ones place. Exchange 10 ones for 1 ten: write the extra ones underneath and carry the 1 ten into the tens column. Example: 8 + 5 = 13 ones = 1 ten and 3 ones. Write 3, carry 1 ten.
Estimate first
Round each number to the nearest 10, then add the rounded numbers. This gives an estimate you can use to check your answer is sensible. Example: 134 + 53 → 130 + 50 = 180, so the real answer should be close to 180.
Worked examples
Add a 3-digit number and a single digit
- Add the ones: 8 + 5 = 13 ones.
- 13 ones is 1 ten and 3 ones, so write 3 and carry 1 ten.
- Bring down the tens and hundreds: 6 tens + 1 carried ten = 7 tens, and 2 hundreds.
Answer: 268 + 5 = 273
Estimate and add a 3-digit and a 2-digit number
- Estimate: 134 rounds to 130, 53 rounds to 50, so 130 + 50 = 180.
- Add the ones: 4 + 3 = 7.
- Add the tens: 3 + 5 = 8. Bring down the 1 hundred.
Answer: 134 + 53 = 187 (close to the estimate of 180)
Estimate and add two 3-digit numbers
- Estimate: 148 rounds to 150, 136 rounds to 140, so 150 + 140 = 290.
- Add the ones: 8 + 6 = 14 ones = 1 ten and 4 ones. Write 4, carry 1 ten.
- Add the tens: 4 + 3 + 1 carried = 8 tens. Add the hundreds: 1 + 1 = 2 hundreds.
Answer: 148 + 136 = 284 (close to the estimate of 290)
Where you see this in real life
- Adding the adults and children who visited a library to find the total number of people.
- Working out how many cakes a baker made by adding two batches together.
- Finding how many books a library has by adding fiction and non-fiction books.
Key words
- Regroup
- To exchange 10 ones for 1 ten (or 10 tens for 1 hundred) when a column has 10 or more.
- Exchange
- To swap 10 of one place for 1 of the next place up, such as 10 ones for 1 ten.
- Estimate
- A close answer found by rounding, used to check your calculation is sensible.
- Ones, tens, hundreds
- The place-value columns: ones on the right, then tens, then hundreds.
- Total
- The answer you get when you add numbers together.