Use compensation
399 + 245 is close to 400 + 245.
Adjust a number to make the addition easier, then balance it.
Year 5
Add and subtract numbers mentally with increasingly large numbers
Use mental strategies such as partitioning, compensation and adjusting with larger numbers.
Lesson overview
Number - addition and subtraction
Mental calculation works well when we see helpful number patterns. We can split numbers into parts, adjust one number to make the calculation easier, and then correct the answer at the end.
Partition to subtract
7,200 - 340 can be 7,200 - 300 - 40.
Subtract in parts when that feels more manageable.
Use a known number fact
25,000 + 75,000 makes 100,000.
Spot pairs that make a tidy total.
Worked example
Calculate 60,000 - 19,800 mentally.
- Think of 19,800 as 20,000 - 200.
- Subtract 20,000 from 60,000 to get 40,000.
- Add back the extra 200.
- The answer is 40,200.
Round 1: Learn it
Look for a mental strategy before reaching for a written method.
Finish this round to unlock Round 2: Show you know it.
Question 1
What is 399 + 245?
Question 2
Calculate 7,200 - 340 mentally.
Question 3
What is 25,000 + 75,000?
Question 4
Calculate 60,000 - 19,800 mentally.
Question 5
Which is the best answer to 48,000 + 2,900?
Common mistakes
Watch for these as you work through the lesson.
- Adjusting a number and forgetting to balance the answer.
- Breaking a number into unhelpful parts.
- Trying to do too much at once instead of one step at a time.
Related topics
These ideas connect closely to this lesson.
- Add and subtract whole numbers with more than 4 digits, including using formal written methods where appropriate.
- Solve addition and subtraction multi-step problems in contexts, deciding which operations and methods to use and why.