Math · Stage 5

Estimate with decimals by rounding to whole numbers

Round each decimal to the nearest whole number first, then add, subtract or multiply the easy whole numbers in your head — a China-fast way to a sensible answer and to catch a silly mistake.

✦ Math📘 Stage 5 · S5⏱ ~12 min🧒 Ages 9–10★ Free to start

I can estimate sums, differences and products of decimals by rounding each decimal to the nearest whole number, then computing quickly in your head.

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Kavi's Lightning Check ⚡

At the night market the stalls shout prices with little decimals — 4.8, 17.9, 3.1. "No need to work it out exactly first," hoots Kavi. "I round each decimal to the nearest whole number, then add, take away or times the easy numbers in my head — a lightning way to see if an answer makes sense ⚡."

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What your child will learn

1Every decimal sits between two whole numbersThe whole numbers are the signposts: 0, 1, 2, 3 …. Every decimal sits between two of them — rounding means hop to the closest whole number. Tap Hop +1 to travel signpost to signpost.

2Round, then add the easy numbersThis is why rounding is so handy. To estimate 24.8 + 17.9, first round each decimal to the nearest whole number: 24.8 → 25 and 17.9 → 18. Now add the easy whole numbers. Tap Step → to build 25 + 18.

3Round the same way, then subtractA difference works exactly the same. To estimate 61.8 − 27.9, round each decimal to its nearest whole number: 61.8 → 62 and 27.9 → 28. Then subtract the easy whole numbers. Tap Step → to build 62 − 28.

4Round, then multiplyTo estimate a product, round each decimal to its nearest whole number, then multiply. For 6.8 × 3.1 round to 7 × 3. That means three 7s — the lit numbers count them out: 7, 14, 21.

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A peek inside

Round 24.8 to the nearest whole number.

tenths digit is 8 (5 or more) → round up

Try this at home: Round to the nearest whole number: 24.8 → __ · 6.2 → __ · 7.5 → __

Ready to play and learn?

Start the free interactive lesson, or print the handout for screen-free play.

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Part of Math · Stage 5 · S5 on KAVI Academy — a secular, whole-child learning platform for ages 5–12.