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Odd one out

All but one share a property — even, a multiple of 7, square, prime, birds — and one does not. Find it; the properties get subtler as you climb.

  • Pattern
  • Timed
  • Endless levels

How it plays

You see a handful of numbers or words. All but one share something; pick the one that does not. Numbers start with even and odd, then multiples, squares and primes, cubes, numbers that read the same backwards, powers of two and digit sums. Words are kinds of thing: birds, metals, sports.

As you climb the numbers grow, the outsider is chosen to look like it belongs (a prime-looking 91, a square-looking 50), and the odd word comes from a nearer kind — a fish among birds rather than a sofa.

Every set of numbers is checked against the whole list of properties: if another property would pick out a different number, the set is thrown away. So there is one answer, and the working says why.

Keys

Space starts and skips ahead · 1–9 pick · Enter submits · Esc stops. On a phone, tap.

What it trains

This practises noticing number properties and kinds of thing quickly. It is practice for odd-one-out questions; it does not raise intelligence.

Learn

How to get better at odd one out: the method, the techniques, what each new level asks of you, and the traps the wrong answers are made of.

Your approach to each question

  1. Read every item before you judge any. The odd one is defined by what the others share, so look for the common thread first.
  2. Run the cheap tests first: even or odd, the last digit, then small multiples (3, 4, 5, 10), then squares, primes and the rest.
  3. When a property fits all but one, check every item against it honestly — the outsider was chosen to look as if it fits.
  4. For words, say the exact kind of each one under your breath: “mammal, mammal, fish, mammal”. The one with a different name is out.

Techniques and tricks

  • Divisibility tests

    3 and 9: the digit sum divides. 4: the last two digits do. 6: even and divisible by 3. 8: the last three digits. 11: the alternating sum of digits. 12: by 3 and 4. 15: by 3 and 5.

    e.g. 261: 2 + 6 + 1 = 9, a multiple of 9. 80: 8 + 0 = 8, so not.

  • Tests for 7 and 13

    For 7, double the last digit and take it from the rest. For 13, add four times the last digit to the rest. Repeat until the number is small enough to know.

    e.g. 91: 9 − 2 = 7, so 91 = 7 × 13. 559: 55 + 36 = 91, a multiple of 13, so 559 is too.

  • Prime pretenders

    Know the odd numbers that look prime and are not: 51 = 3 × 17, 57 = 3 × 19, 87 = 3 × 29, 91 = 7 × 13, 119 = 7 × 17, 133 = 7 × 19, 143 = 11 × 13. To test a prime, try divisors only up to its square root.

    e.g. 113: try 2, 3, 5, 7 — none divides, and 11² = 121 is already bigger. So 113 is prime.

  • Squares and cubes by heart

    Squares to 20² = 400 and cubes to 10³ = 1,000. A number between two squares is not a square.

    e.g. 50 lies between 7² = 49 and 8² = 64. 8 among 9, 81, 100 and 196 is a cube, but not a square.

  • Halve until odd

    For powers of 2 (1, 2, 4 … 512, 1,024), halve the number until it is odd. Ending at 1 means a power of 2; ending at anything else means not.

    e.g. 96 → 48 → 24 → 12 → 6 → 3. So 96 = 32 × 3, not a power of 2.

  • Read from both ends

    For numbers that read the same backwards, pair the first digit with the last and work inwards. One mismatched pair is enough.

    e.g. 4,554 pairs 4–4 and 5–5; 4,555 pairs 4–5 at once.

  • Sum the digits of each

    When nothing else separates them, add the digits of every number. The one with a different total is out.

    e.g. 238, 346 and 517 all add to 13; 248 adds to 14.

As the levels climb

  • Numbers start with even and odd; the odd word comes from a far kind, such as a landscape among mammals.
  • Multiples of 3, 4, 5 and 10 come next, then bigger numbers, then the odd word comes from a near kind — a fish among mammals, a kitchen thing among furniture — then 80% of the time.
  • Then multiples of 6 to 9, squares and primes; bigger numbers; 65% time. Then multiples of 11 and 12, cubes and same-backwards numbers; bigger numbers; 55% time. Last come multiples of 13 and 15, powers of 2 and digit sums. After that only the numbers grow.
  • With “Questions about” on Words alone there are five levels: far kinds, near kinds, then the clock at 80%, 65% and 55%.

Traps to avoid

  • The outsider built to belong: a prime-looking 91 or 57 among primes, a number a few off a multiple (110 among multiples of 15). Test by dividing, not by the look of it.
  • Near squares and cubes: n × (n + 1) beside squares (42 beside 36 and 49), a square ± 1 or 2, or one more than a cube (28 among 8, 27 and 64). Check against the list you know.
  • The one that is merely unusual. Being the only multiple of 7, or the only number over 100, does not make it the odd one. The odd one lacks what all the others share.
  • The near kind among words. A fish among mammals, or a vegetable among fruit, passes a quick “animal” or “food” check. Name the exact kind.

How to practise

  • Set “Questions about” to Numbers to drill the properties alone — that ladder never ends — or to Words for the five-level kinds ladder.
  • Raise “Items shown” to 7 for more checks per question; lower it to 4 while a new property is still slow.
  • After a miss, read the working: it shows the division or the squares that rule the outsider out. Add any new prime pretender to the list you keep.

Levels

Each question has a clock that comes from the work it asks for. There is no last level: at least one dial turns without end. Your level is judged on your last ten answers, every ten: 90% moves you up, below 80% moves you down, which holds most people near four in five right.

All of it is in the settings beside Start — how many answers a judgement looks at and how often it recalibrates, the thresholds, the time, which questions, how many options, and whether you see each verdict at once or review them together at the judgement. You can also jump to any level.

More pattern

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