Estimate with friendly numbers
248 is close to 250 and 149 is close to 150, so 248 + 149 is about 250 + 150, which is about 400.
An estimate does not have to be exact. It helps us judge the size of the answer.
Year 3
Estimate the answer to a calculation and use inverse operations to check answers
Estimate sensible answers and use inverse operations to check calculations.
Lesson overview
Number - addition and subtraction
Estimating helps us decide whether an answer is reasonable. We can think about how big the answer should be, using friendly numbers close to the real ones, to make a quick guess in our heads. We can also use the inverse operation to check, such as subtracting to check an addition.
Use subtraction to check addition
If 326 + 143 = 469, then 469 - 143 should equal 326.
Inverse operations undo each other.
Use addition to check subtraction
If 500 - 178 = 322, then 322 + 178 should equal 500.
The check should take us back to the starting number.
Worked example
Check whether 287 + 201 = 488 is reasonable.
- Estimate the answer first: 287 is close to 300 and 201 is close to 200, and these are easy to add in your head.
- 300 + 200 is about 500.
- 488 is close to 500, so it sounds reasonable.
- Use subtraction to check exactly: 488 - 201 = 287.
Try it
Estimate first, then think about which inverse check matches the calculation.
Question 1
What is a sensible estimate for 198 + 103?
Question 2
Which inverse check matches 345 + 120 = 465?
Question 3
Which estimate is best for 612 - 291?
Question 4
Use the inverse to check: if 520 - 180 = 340, then 340 + 180 =
Question 5
Why do we estimate first?
quick sensible guess
careful full calculation
Common mistakes
Watch for these when working through the lesson.
- Treating the estimate as the exact answer.
- Using the wrong inverse operation.
- Choosing friendly numbers that are not close enough to the real numbers, or that are not easy to work with.
Related topics
These ideas fit closely with this lesson.
- Add and subtract numbers mentally, including a three-digit number and ones, tens and hundreds.
- Solve problems, including missing number problems, using number facts, place value, and more complex addition and subtraction.