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Place Value to 100,000 & Rounding

Maths · Y4 OC Prep
Year 4 OC
Lesson Guide
Explanation
OC questions use numbers up to five digits, so meet the ten-thousands column: 36249 is 3 ten-thousands, 6 thousands, 2 hundreds, 4 tens, 9 ones. A typical OC question asks: 'What number is seven hundreds less than 36249?' Translate the words — seven hundreds means 700 — then subtract: 36249 − 700 = 35549. Watch the borrow: the 2 hundreds can't give 7, so a thousand breaks open.
Your Turn
OC style: what is four thousands more than 58792? And three tens less than 40013? Check: 62792; and 39983 (the tens borrow across the zero!).
Watch Out
'Seven hundreds' means 700, not 7. Translating quantity-words into digits is half of every OC place-value question.
Explanation
You know digit-card games from Year 3 — OC adds a second step. OC style: cards 7, 3, 8 make three-digit numbers; find the DIFFERENCE between the largest and smallest you can build. Build both (largest 873, smallest 378), then subtract: 873 − 378 = 495. OC questions almost never stop at step one — after you build, expect to compare, add or subtract.
Your Turn
Cards 7, 1, 5: largest, smallest, and their difference. Check: 751, 157, and 751 − 157 = 594.
Watch Out
If a 0 card appears, the smallest number cannot START with it — 0 goes second (like 1047 from 0,1,4,7).
Explanation
You can round — OC tests whether you understand what rounding DOES. OC style: round 35, 62, 88, 70 and 91 to the nearest 10 — which changes the most? Work out how far each one MOVES: 35→40 moves 5, 62→60 moves 2, 88→90 moves 2, 70 stays, 91→90 moves 1. Winner: 35. Two insights: numbers ending in 5 round UP and travel the maximum distance, and numbers already ending in 0 don't move at all.
Your Turn
Rounded to the nearest 100, which moves furthest: 149, 250, 301, 490? Check: 250 → 300 moves 50 — the halfway number wins again.
Watch Out
'Changes the most' means the move distance, not the biggest result. Read what the question is really asking.
Explanation
In the OC test you get about 68 seconds per question — estimation is a weapon. 387 + 412: round to 400 + 400 = about 800. Now look at the options: 599, 699, 799, 899, 999 — only 799 lives near 800. Answer found without exact working! Round, estimate, eliminate. Even when two options survive, you've halved the work.
Your Turn
Estimate to pick: 61 × 5 is closest to… 200, 300, or 500? Check: 60 × 5 = 300.
Watch Out
Estimation picks between spread-out options. If options are close together (798, 799, 800), you must compute exactly.
Explanation
Practice-run through the three OC place-value styles: word-quantities ('three thousands more than 98250' = 101,250 — notice it crossed 100,000!), digit-card building with the zero rule (smallest from 8,0,3,5 is 3058), and rounding in REVERSE ('rounds to 400 — what's the largest it could be?' — 449, since 450 would round up to 500). That reverse-rounding idea is pure OC: they love running your skills backwards. Quiz and exercises — in real five-option format!
Your Turn
Reverse rounding: a number rounds to 70 (nearest 10). What's the smallest it could be? Check: 65.
Watch Out
The largest number that rounds DOWN to 400 ends in 49, not 50 — the halfway point 450 always rounds UP.
Step 1: Introduction
Step 1 of 5
36249 — five columns now 3 ten-thousands 6 thousands 2 hundreds 4 tens 9 ones OC question style: "What number is seven hundreds less than 36249?" Seven hundreds = 700. 36249 − 700 = 35549. Only the hundreds (and sometimes thousands) digits change!
OC style: cards 7, 3, 8 7 3 8 largest: 873 smallest: 378 difference: 873 − 378 = 495 OC never stops at "build the largest" — it asks for the DIFFERENCE. Step 1 build both. Step 2 subtract. Always expect the second step.
Which number changes the MOST? Round each to the nearest 10 — which number changes the most? 35 → 40 moves 5 62 → 60 moves 2 88 → 90 moves 2 70 → 70 moves 0 91 → 90 moves 1 The question isn't "round these" — it's "how FAR does each move?" 35 → 40 moves 5 (halfway rounds UP). 70 doesn't move at all. Numbers ending in 5 move the furthest of all.
Estimate first — kill wrong options 387 + 412 = ? estimate: 400 + 400 = about 800 A 599 B 699 C 799 D 899 E 999 With ~68 seconds per question, estimation is your speed tool: 1. Round the numbers. 2. Rough answer. 3. Cross out impossible options. Often only ONE option survives — no exact working needed! Here only C is near 800. Done in ten seconds.
Put it together 🎯 "What number is three thousands more than 98250?" 98250 + 3000 = 101,250 — it crossed 100,000! "Cards 8, 0, 3, 5: build the smallest 4-digit number." Can't start with 0 → 3 first, then smallest: 3058. "I round a number to the nearest 100 and get 400. What is the largest the number could be?" 449 — anything from 350 up to 449 rounds to 400.
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