GRADE 3 · ALBERTAOUTCOME AB-MAT3-N2
L4 · Estimating sums and differences
Learning goal
“I can estimate a sum or difference by rounding both numbers to the nearest hundred. Then I can judge whether an answer is reasonable.”
Estimate — a quick answer found with rounded numbers. It tells about how big the real answer should be.
Round — to replace a number with the nearest ten or hundred (from G3U01).
Reasonable — an answer is reasonable if it lands close to a good estimate. Far away means something went wrong.
About — the word that travels with estimates: "about 700," never an exact-looking number.
Worked example
Estimate 384 + 257. Step 1 (round both to the nearest hundred): 384 -> 400. 257 -> 300. Step 2 (add the rounded numbers): 400 + 300 = 700. The sum is about 700. Step 3 (compare with exact): the standard algorithm gives 641 - near 700. Reasonable.
Check yourself — explain it
We round both numbers before estimating because...
Paired problem — example still visible
Estimate 512 + 378: 512 -> 500; 378 -> 400; estimate -> 900 (exact: 890)
Faded completion — last step is yours
"Estimate 693 + 218: 693 -> 700; 218 -> 200; 700 + 200 = ___" -> 900 (exact: 911)
Ready to practise?
10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.