Concept 9 of 9
Estimate to Check
Today's big idea
Round each number to the nearest ten first, then add to get an 'about' answer that checks your exact one.
38 + 41 rounds to 40 + 40 = about 80.
How it works
👀 What's going on?
Before you add 38 and 41 exactly, you can get a quick about answer. Round each number to the nearest ten and add those, and you will know roughly what the real answer should be.
💡 Here's the trick
Look at the rounded numbers above. 38 is close to 40 and 41 is close to 40, so the answer is about 40 + 40 = 80. Now you know the exact answer should land near 80. If you worked it out and got 8 or 800, you would spot at once that something went wrong.
🪄 How to do it
- 1.Round each number to the nearest ten (5 or more in the ones rounds up).
- 2.Add the tidy tens.
- 3.Use that estimate to check your real answer is close.
✍️ Try it
Estimate 52 + 29 to the nearest ten. Then estimate 63 − 38.
52 rounds to 50 and 29 rounds to 30, so about 80. And 63 rounds to 60 and 38 rounds to 40, so about 20.
⚠️ Watch out
An estimate is not the exact answer. About 80 does not mean exactly 80; it means the real answer should be near 80, not miles away.
🌟 Why this matters
A quick estimate catches big mistakes before they cost you marks. It is the fastest way to check your own work.
Exam tip: If your answer is far from your estimate, work it out again.
Watch me solve one
Quanta solves it
“Watch how I think through one of these — then you try.”
THE PROBLEM
“Estimate 38 + 41 by rounding to the nearest ten.”
- 1
Round each number
38 → 40, 41 → 40.
- 2
Add the tidy tens
40 + 40 = 80
- 3
Answer ✓
about 80
Now you try one — same steps, different story 👇
See it in action
⚡ Time to play!
Read the story and use the clue word.
23 birds sit. 18 more fly in all. How many altogether?
Does “in all” mean add or subtract?
Quick check
✅ VERIFICATION
Estimate 52 + 29 by rounding each number to the nearest ten.
⚡ Finish the activity above first to answer.
Keep going!