L3 · Estimation: is the answer reasonable?

Learning goal

“I can estimate a sum or difference by rounding the numbers first, and use my estimate to judge whether a calculated answer is reasonable - before I trust it.”

Estimate — a quick rough answer found with rounded numbers. An estimate is close, not exact.

Reasonable — an answer is reasonable if it sits close to the estimate and makes sense in the problem.

Round first — the estimation method: round each number to a chosen place, then add or subtract the rounded numbers.

Worked example

Estimate 4,812 + 3,095. Round each number to the nearest hundred: 4,812 -> 4,800 and 3,095 -> 3,100. Add the rounded numbers: 4,800 + 3,100 = about 7,900. (Exact answer, for comparison: 7,907. The estimate missed by only 7.) Say it plainly: rounding to a FINER place (hundreds instead of thousands) gives a closer estimate. Both are useful - the job of an estimate is to be quick and close, not exact.

Check yourself — explain it

An estimate is allowed to be a little wrong because...

Paired problem — example still visible

Estimate 689 + 2,914 by rounding to the nearest hundred -> 700 + 2,900 = about 3,600 (exact 3,603)

Faded completion — last step is yours

"Estimate 5,236 - 1,842 by rounding to the nearest thousand: 5,236 -> ___, 1,842 -> ___, so the difference is about ___." -> 5,000; 2,000; 3,000

Ready to practise?

10 scored questions with a 3-rung hint ladder. Mastery needs 8/10 plus both gate questions.

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