Maths Olympiad · topic 7 of 33 · free

Odd one out — Maths Olympiad practice

In an odd-one-out question, three numbers follow the same rule and one breaks it. Here all four numbers are odd (or all even), so you have to think harder: which times table do three of them belong to?

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How to solve it

Find the shared rule — “look for a table that three of the numbers belong to”

Check whether all the numbers are odd or all even — if so, look deeper.
Try the tables 3, 4, 5, 6, 7, 8 and 9: which one has three of the numbers in it?
The number that is not in that table is the odd one out.

Use it for: Odd-one-out, classification and “which does not belong” questions.

Worked example: Which is the odd one out: 21, 35, 45, 49? → 45

All four are odd, so look for a table: 21 = 7 × 3, 35 = 7 × 5, 49 = 7 × 7.
45 is not in the table of 7 (45 ÷ 7 = 6 remainder 3).

Check: 7 × 6 = 42 and 7 × 7 = 49, so 45 falls between them and is not in the table of 7 ✓

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✓ Written and checked by the TalentJR team · Updated 13 Oct 2026 · How we make our lessons

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