More addition & subtraction
Adding and subtracting efficiently
We can add and subtract whole numbers quickly using the column method. We line up the ones, tens and hundreds, then add or subtract each column, carrying or regrouping when we need to.
Estimate first
Before calculating, round each number to the nearest 100 and estimate the answer. This helps you check afterwards that your answer is reasonable. For example, 456 + 789 is about 500 + 800 = 1300.
Adding in columns
Write the numbers with ones under ones, tens under tens and hundreds under hundreds. Add each column, starting with the ones. If a column adds to 10 or more, write the ones digit and carry the ten to the next column.
Subtracting in columns
Line up the place values and subtract each column, starting with the ones. If you cannot subtract a column, regroup: change 1 hundred into 10 tens, or 1 ten into 10 ones, then subtract.
Worked examples
Adding with carrying
- Line up 318 and 425.
- Ones: 8 + 5 = 13, write 3 carry 1.
- Tens: 1 + 2 + 1 = 4.
- Hundreds: 3 + 4 = 7.
Answer: 318 + 425 = 743
Subtracting with regrouping
- Line up 367 and 185.
- Ones: 7 − 5 = 2.
- Tens: you cannot do 6 − 8, so regroup 1 hundred into 10 tens: 16 tens − 8 tens = 8 tens.
- Hundreds: 2 − 1 = 1.
Answer: 367 − 185 = 182
Estimating to check
- To check 456 + 789, round each number to the nearest 100.
- 456 is about 500 and 789 is about 800.
- 500 + 800 = 1300, which is close to the exact answer 1245.
Answer: The estimate 1300 shows 1245 is reasonable.
Where you see this in real life
- Working out how much lunch costs in a cafe by adding the prices.
- Finding how many bricks a builder has left after using some.
- Adding the number of adults and children at a concert.
- Checking change by subtracting a price from the money you paid.
Key words
- Sum
- The answer when you add numbers together.
- Difference
- The answer when you subtract one number from another.
- Carry
- Moving a ten (or hundred) into the next column when a column adds to 10 or more.
- Regroup
- Exchanging 1 hundred for 10 tens, or 1 ten for 10 ones, so you can subtract.
- Estimate
- A close answer found by rounding, used to check a calculation.