Multiplying by 10, 100 & 1000

Practise the place-value shortcut for multiplying quickly by powers of ten — a key mental-math skill.

Grades 4–5 · 5.NBT⚡ Mental math
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How multiplying by tens works

Multiplying by 10, 100 or 1000 is the fastest mental-math shortcut there is — once a child sees what is really happening. Every time you multiply by 10, each digit shifts one place to the left, into a column ten times bigger. For whole numbers that shows up as adding a zero, but the real rule is the place-value shift, which keeps working for decimals later.

  1. Count the zeros: 10 has one, 100 has two, 1000 has three.
  2. Shift every digit that many places to the left (for whole numbers, add that many zeros).
  3. For something like 7×40, split it: do 7×4 first, then shift for the ×10.

Worked examples

Multiplying by a multiple of ten7 × 40 — 7×4 = 28, then ×10 shifts it: 280.
Multiplying by a multiple of a hundred6 × 300 — 6×3 = 18, then ×100 shifts it two places: 1,800.
AD AREA (parent reading zone only — never shown during practice)

Tips & common mistakes

Teach it as place-value shifting, not “just add zeros” — the zero rule breaks the moment decimals appear (3.5×10 = 35, not 3.50), so getting the idea right now saves confusion later. The most common mix-up is between ×10 and ×100, which is exactly why the answer choices here include both 28 and 2800 for a question like 7×40 — read the question carefully.

  • Counting the wrong number of zeros (one for 10, two for 100, three for 1000).
  • Relying on “add a zero” — fine for whole numbers, wrong for decimals.
  • Confusing ×10 with ×100 — off by a factor of ten.

Frequently asked questions

Why does multiplying by 10 add a zero?

Because every digit shifts one place to the left, which for a whole number shows up as a zero on the end. It is really place value, not a trick.

Does the “add a zero” rule always work?

No — it fails for decimals (3.5×10 = 35, not 3.50). Thinking of it as shifting each digit one place left always works.

What grade is this?

It is practised in grades 4–5 and underpins a great deal of later mental math and work with decimals.

How does this help with bigger multiplication?

It is the backbone of long multiplication — the placeholder zero when you multiply by the tens digit is exactly this shortcut at work.

What about multiplying by 20, 30, 40?

Split it: 7×40 is 7×4 then ×10. Do the easy fact first, then apply the shift.

Keep practising

← All multiplicationTimes tablesMulti-digit5th grade math

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