N4.4 Stage 4 Number

Multi-digit ×/÷ & divisibility

Multiply 2–3-digit by 1-digit (formal written / area model); short/Euclidean division with a 1-digit divisor & remainders; factor pairs; divisibility rules (2, 3, 5, 10); the distributive law.

By now you know your times tables by heart. This concept scales that up: multiplying and dividing numbers too big to just remember, and spotting the hidden structure of a number through its factors and divisibility.

What it means

A multi-digit multiplication like 243 × 7 is too large to recall, so we break it into pieces we can do. The key idea is the distributive law: multiplying a sum is the same as multiplying each part and adding the results. Because 243 = 200 + 40 + 3, we can write

243 × 7 = (200 × 7) + (40 × 7) + (3 × 7) = 1400 + 280 + 21 = 1701.

The area model pictures this: a rectangle 243 wide and 7 tall, sliced into three columns of area 1400, 280 and 21. The formal written method (long multiplication) is the same sum, just stacked in columns with carrying.

Division undoes multiplication. 1701 ÷ 7 asks “how many 7s fit in 1701?” — the answer is 243 exactly, since we just built it. When it does not divide evenly, a remainder is left over: 17 ÷ 5 = 3 remainder 2, because 5 × 3 = 15 and 2 is left. This is Euclidean division: any whole number split by a divisor gives a quotient and a remainder smaller than the divisor.

A factor of a number is a whole number that divides it with no remainder. Factors come in pairs that multiply to give the number: for 12 the pairs are 1 × 12, 2 × 6, 3 × 4. A number is divisible by another when the remainder is zero, and there are quick divisibility rules to check without dividing:

  • by 2 — the last digit is even (0, 2, 4, 6, 8)
  • by 5 — the last digit is 0 or 5
  • by 10 — the last digit is 0
  • by 3 — the digits add up to a multiple of 3

Worked examples

Area model for 36 × 4:

×306
412024

Add the parts: 120 + 24 = 144.

Short (Euclidean) division 95 ÷ 4:

95 ÷ 4
 9 ÷ 4 = 2 remainder 1   (carry the 1 -> 15)
15 ÷ 4 = 3 remainder 3
Answer: 23 remainder 3   (check: 4 × 23 = 92, plus 3 = 95)

Divisibility check on 45: last digit is 5, so divisible by 5. Digits 4 + 5 = 9, a multiple of 3, so divisible by 3. Not even, so not divisible by 2. Its factor pairs are 1 × 45, 3 × 15, 5 × 9.

The generative-art connection

An array of dots is a multiplication: 4 rows of 30 dots is 4 × 30, and the area model is that same rectangle recoloured by its parts. The dot-multiplier tool literally grows one dot into a symmetric array — you are watching a product take shape.

Factors and remainders make patterns too. Mark points evenly around a circle, number them, and join each point n to point 2n (its double). As you sweep through all the multipliers, the times-table strings trace a glowing cardioid — a heart-shaped curve built entirely from multiplication (The Coding Train, challenge #133). Divisibility is the rhythm behind it: which points land back on themselves, and how the leftovers (remainders) cycle, decides the shape.

Common misconceptions

  • Dropping the place value. In 243 × 7, the middle step is 40 × 7 = 280, not 4 × 7 = 28. The area model keeps the zeros visible so this stays honest.
  • Writing a remainder as a decimal by accident. 17 ÷ 5 = 3 remainder 2 is not the same as 3.2 at this stage — a remainder is a whole leftover, not a fraction of the divisor.
  • Confusing factors with multiples. Factors divide into a number (3 is a factor of 12); multiples are what you get by multiplying (24 is a multiple of 12). They point in opposite directions.

What a learner should be able to solve

The benchmark for mastering this concept — the problems a child at this stage is expected to be able to work through.

  1. Work out `487 × 6` using the formal written method (or the area model). Show your steps.

    Answer

    `2922`. By parts: `400 × 6 = 2400`, `80 × 6 = 480`, `7 × 6 = 42`; total `2400 + 480 + 42 = 2922`.

    Art hook Draw a rectangle 487 wide and 6 tall, then slice it into 3 stacked colour bands (2400, 480, 42) whose widths on a shared bar are proportional to each part. Watching the bands add up to the full bar shows the distributive law visually.

  2. A rope `238` cm long is cut into `7` equal pieces. How long is each piece? Then try a `239` cm rope and give the remainder.

    Answer

    `238 ÷ 7 = 34` cm exactly (`7 × 34 = 238`). `239 ÷ 7 = 34 remainder 1` (one extra cm left over).

    Art hook Draw a horizontal number line 0 to 238 and drop 7 evenly-spaced tick marks; light up each 34-length segment in a rotating hue. Add 1 cm to make 239 and a tiny leftover stub appears at the end, coloured differently to show the remainder.

  3. List every factor of `24` by finding its factor pairs.

    Answer

    Pairs: `1 × 24`, `2 × 12`, `3 × 8`, `4 × 6`. Factors: `1, 2, 3, 4, 6, 8, 12, 24`.

    Art hook For a number N, place N dots and try to arrange them into every possible rectangle grid (1×24, 2×12, 3×8, 4×6). Each valid rectangle is a factor pair; morph between them so the dots flow from one grid shape into the next.

  4. Test `2730` for divisibility by 2, 3, 5 and 10 using the rules — no long division allowed.

    Answer

    Last digit `0`: divisible by 2, by 5 and by 10. Digit sum `2 + 7 + 3 + 0 = 12`, a multiple of 3: divisible by 3. So `2730` is divisible by all four.

    Art hook A colour-wheel dashboard: four arcs (2, 3, 5, 10) that light green or stay grey as you type a number. The digit-sum test animates by adding the digits one at a time along a small orbit, then checking if the total is a multiple of 3.

  5. Use the fact that `56 × 8 = 448` to find `448 ÷ 8` and `448 ÷ 56` without dividing. Explain how you know.

    Answer

    `448 ÷ 8 = 56` and `448 ÷ 56 = 8`. Multiplication and division are inverses, so a known product gives two division facts for free.

    Art hook Show one array of 448 dots as an 8-by-56 grid. A toggle relabels the same grid as '8 groups of 56' or '56 groups of 8', so the child sees the product and both quotients in a single unchanging picture.

Training exercises

Practice problems, easier first, that build toward the bar above. Each doubles as a seed for an interactive artwork.

  1. Skip-count to help: what is `5 × 60`? And `50 × 6`?

    Answer

    Both are `300`. `5 × 6 = 30`, then attach the extra zero for the ten.

    Art hook Place 5 dots, then multiply each into a column of 60 (or 50 dots into 6 columns). The two arrangements fill the same 300-dot rectangle, just rotated — a first glimpse of commutativity.

  2. Fill in the area model for `53 × 4`: work out the box for `50 × 4` and the box for `3 × 4`, then add them.

    Answer

    `50 × 4 = 200`, `3 × 4 = 12`; total `200 + 12 = 212`.

    Art hook A 2-cell rectangle you can drag to resize: one cell always 4 tall and 50 wide, the other 4 tall and 3 wide. The cell areas print live and their colours blend into the total.

  3. Work out `274 × 3` with the formal written method.

    Answer

    `822`. `200 × 3 = 600`, `70 × 3 = 210`, `4 × 3 = 12`; `600 + 210 + 12 = 822`.

    Art hook Three coloured tiles of size 600, 210, 12 slide together onto a number line, stacking end to end until they reach 822.

  4. Share `84` stickers equally among `6` children. How many each? Any left over?

    Answer

    `84 ÷ 6 = 14` each, remainder `0` (`6 × 14 = 84`).

    Art hook Deal 84 dots one at a time into 6 orbiting bins around a circle; the counter under each bin ticks up to 14 in sync, showing the share growing evenly.

  5. Do the short division `137 ÷ 4` and give the answer with its remainder. Then check your answer.

    Answer

    `34 remainder 1`. `13 ÷ 4 = 3 r 1` (carry 1 → 17), `17 ÷ 4 = 4 r 1`. Check: `4 × 34 = 136`, `+1 = 137`.

    Art hook Animate 137 dots falling into 4 equal columns; 136 fill the columns evenly and the single leftover dot glows alone at the top as the remainder.

  6. Find all the factor pairs of `18`, then all the factor pairs of `16`. Which number has an odd count of factors, and can you guess why?

    Answer

    `18`: `1×18, 2×9, 3×6` → 6 factors. `16`: `1×16, 2×8, 4×4` → 5 factors (`4×4` counts once because both factors are the same). Square numbers have an odd count because one pair is a number times itself.

    Art hook Arrange each number as every rectangle it can make. For 16 one rectangle is a perfect 4×4 square — highlight square numbers in gold to reveal why their factor count is odd.

  7. Sort these into divisible-by-3 and not divisible-by-3: `27`, `40`, `114`, `205`. Use the digit-sum rule.

    Answer

    `27` (2+7=9) yes; `40` (4+0=4) no; `114` (1+1+4=6) yes; `205` (2+0+5=7) no.

    Art hook A grid of numbered dots where every multiple of 3 lights up when the '÷3' rule is switched on, revealing a diagonal stripe pattern across the grid.

  8. Spot the mistake: a pupil works out `216 × 4` by multiplying each digit as if it were just units — `2 × 4 = 8`, `1 × 4 = 4`, `6 × 4 = 24` — and writes a small wrong answer. What went wrong, and what is the correct answer?

    Answer

    They ignored place value: the `2` means `200` and the `1` means `10`, not `2` and `1`. Correct: `200 × 4 = 800`, `10 × 4 = 40`, `6 × 4 = 24`; total `800 + 40 + 24 = 864`.

    Art hook Two area rectangles side by side: the wrong one loses the zeros and shrinks tiny; the correct one keeps 800/40/24 full-size, so the eye sees how much value was thrown away.

  9. A baker packs `150` buns into boxes of `8`. How many full boxes, and how many buns are left over?

    Answer

    `150 ÷ 8 = 18 remainder 6`. So 18 full boxes and 6 buns left. Check: `8 × 18 = 144`, `+6 = 150`.

    Art hook 150 dots stream into boxes of 8 arranged in a grid; 18 boxes fill and seal, and 6 loose dots hover in a leftover tray labelled 'remainder'.

  10. Use `18 × 5 = 90` to work out `18 × 6` in your head (no full multiplication).

    Answer

    `108`. `18 × 6 = 18 × 5 + 18 = 90 + 18`.

    Art hook A bar of length 90 (five 18-blocks) with a sixth 18-block sliding on the end; the total readout climbs from 90 to 108 as the block clicks into place — the distributive law in motion.

  11. Reasoning: `240 ÷ 6 = 40`. Without dividing again, find `240 ÷ 12` and explain using factors.

    Answer

    `240 ÷ 12 = 20`. Dividing by 12 is dividing by 6 then by 2, and `40 ÷ 2 = 20`. (Or: 12 is twice 6, so the quotient halves.)

    Art hook Split 240 dots into 6 groups, then let each pair of groups merge into one — the 6 groups become 12 half-size groups, and the count-per-group readout halves from 40 to 20.

  12. Open-ended: find a 3-digit number that is divisible by 2, by 3 and by 5 all at once. How can you be sure without long division?

    Answer

    Any multiple of 30 works, e.g. `120`, `330`, `450`, `990`. It must end in 0 (so divisible by 2, 5 and 10) and have a digit sum that is a multiple of 3.

    Art hook A number line of the 3-digit range where numbers passing all three divisibility rules bloom into flowers; the flowers land exactly on the multiples of 30, spaced evenly like beads.