Odd one out for AFCAT: word, number and letter classification

Classification questions, usually called "odd one out", give you four or five items. All but one share a property, and you pick the one that does not. The items can be words, numbers, letter groups or figures.

They look simple, and many are. The marks are lost when a candidate spots a property that isolates one item, picks it, and misses the stronger property the setter intended. A clear method and a checklist of properties fix that.

The method

1. Guess a property from the first two or three items. Aircraft? Primes? Letters two apart?
2. Test it on every item. Count how many share it.
3. Accept it only if exactly one item fails. If two fail, or none, the property is wrong. Try the next one on your checklist.
4. Check for a second property. If another property also isolates exactly one item, see the section on two possible answers below.
5. Confirm with the options. The answer must be one of them.

Why it works: the question promises that the other items form a group. A property that leaves exactly one out is a candidate for that group. Testing every item stops you from stopping too early.

Words: what to check

• Category: fruit, planet, aircraft, tank, river, instrument.
• Function: what the thing is used for, such as measuring, cutting or carrying.
• Place or habitat: lives in water, grows underground, found in space.
• Part of the same whole: parts of an aircraft, parts of a flower.
• Word form: palindromes, the same suffix, the same number of letters. Check these only after meaning-based properties.

AFCAT often uses defence items, and the trap is usually a close category: a helicopter among fighters, or a tank among missiles. These groups are worth knowing:

Numbers: a checklist

Letters: a checklist

Write A = 1 to Z = 26. The anchors E = 5, J = 10, O = 15, T = 20 and Y = 25 make positions quick.

• Gap between letters: BD skips one letter, BE skips two.
• Opposite letters: positions add up to 27 (A and Z, M and N).
• Vowels and consonants.
• Consecutive letters: BCD, PQR.
• Position patterns: all even positions, all multiples of 5.

Worked examples

Example 1 (words). Barometer, Thermometer, Stethoscope, Hygrometer, Anemometer.

• The first two are both used to measure weather conditions. Test that on each: hygrometer (humidity) yes, anemometer (wind speed) yes, stethoscope no.
• Answer: Stethoscope, a medical instrument for listening. Its name also lacks "-meter", which agrees.

Example 2 (primes). 2, 3, 5, 9, 11.

• Test "prime": 2, 3, 5 and 11 are prime; 9 = 3 × 3.
• Answer: 9.

Example 3 (number pairs). 3–9, 4–16, 5–25, 6–30.

• Rule from the first pair: the second number is the square of the first. 4² = 16 and 5² = 25, but 6² = 36.
• Answer: 6–30.

Example 4 (digit sum). 124, 232, 313, 421, 611.

• Parity does not work: three odd and two even. Primes do not work either, because 124, 232 and 611 (13 × 47) are all composite.
• Digit sums: 7, 7, 7, 7 and 8.
• Answer: 611.

Example 5 (opposite letters). AZ, CX, EV, GS.

• A (1) + Z (26) = 27, C (3) + X (24) = 27, E (5) + V (22) = 27, G (7) + S (19) = 26.
• The first three are opposite-letter pairs. Answer: GS.

Example 6 (word form). Level, Radar, Civic, Rotor, Pilot.

• No meaning connects the first four. Look at the words themselves: level, radar, civic and rotor read the same backwards.
• Answer: Pilot, the only one that is not a palindrome.

When two answers seem to fit

Take 16, 25, 36, 48, 64. "Perfect square" isolates 48. "Even" isolates 25. Both properties leave exactly one out.

Here is how to decide:

• Check the options first. Often only one of the two candidates is offered.
• Prefer the rarer property. Any group of numbers splits roughly half odd and half even, so four even numbers can happen by chance. Four perfect squares in a row almost never happen by chance, so that is the rule the setter chose. The answer is 48.
• Prefer meaning over spelling for words. A category like "aircraft" beats "has six letters".

Practice set

1. Sukhoi, Rafale, Tejas, Arjun
2. 8, 27, 64, 100, 125
3. 13, 17, 21, 23, 29
4. CE, GI, KM, PS
5. 17–289, 13–169, 11–121, 14–186
6. Wing, Cockpit, Fuselage, Runway, Tail
7. 42, 63, 84, 105, 116
8. ACE, BDF, GIK, MOP

Answers:

1. Arjun. It is a main battle tank; the other three are fighter aircraft.
2. 100. The others are cubes of 2, 3, 4 and 5; 100 is 10², not a cube.
3. 21. It equals 3 × 7. The others are prime.
4. PS. C to E, G to I and K to M each skip one letter. P to S skips two (Q and R).
5. 14–186. In each other pair the second number is the square of the first. 14² is 196, not 186.
6. Runway. The others are parts of an aircraft; a runway is part of an airfield.
7. 116. The others are multiples of 21 (21 × 2, 3, 4, 5). 116 ÷ 21 is not a whole number.
8. MOP. In the others, each letter is two places after the one before (A, C, E). In MOP, O to P is only one place.

What to do next

• Learn squares to 30, cubes to 15 and primes up to 100, so number questions are instant.
• Practise letter positions both ways until the anchors (E, J, O, T, Y) are automatic.
• Pair this with verbal analogy and number and letter series, which use the same properties.