Three of the following four options are alike in a certain way and thus form a group. Which is the one that does NOT belong to that group? (NOTE: Operations should be performed on the whole numbers, without breaking down the numbers into its constituent digits. E.g. 13 – Operations on 13 such as adding/subtracting/multiplying etc. to 13 can be performed. Breaking down 13 into 1 and 3 and then performing mathematical operations on 1 and 3 is not allowed.)

Correct — B. Rule: third = first + (second / 2).

36, 88, 80: 88 / 2 = 44, and 36 + 44 = 80 OK
34, 56, 62: 56 / 2 = 28, and 34 + 28 = 62 OK
23, 56, 51: 56 / 2 = 28, and 23 + 28 = 51 OK

68, 54, 94: 54 / 2 = 27, and 68 + 27 = 95, but the triad ends in 94.

That triad is the one outside the group -> option (b).

Why the others are wrong

• (a)
  36 – 88 – 80 — 36 + (88 / 2) = 36 + 44 = 80, which is exactly its third number. This triad obeys the rule and stays in the group.
• (c)
  34 – 56 – 62 — 34 + (56 / 2) = 34 + 28 = 62, exactly its third number. It belongs to the group.
• (d)
  23 – 56 -51 — 23 + (56 / 2) = 23 + 28 = 51, exactly its third number. It belongs to the group.

An odd-one-out triad is a fit test, not a puzzle. Find one relation three sets obey exactly, then measure the fourth against it.

Try halves and doubles early. The bracketed note bars splitting a number into its digits, which leaves +, -, x and / working on the whole numbers themselves.

Once third minus first = second / 2 has held three times, one subtraction settles the answer.

Key facts

• In three of these triads the third number is the first plus half the second.
• The bracketed rider bars digit-splitting, so 36 stays 36 and never becomes 3 and 6.
• 68, 54, 94 misses by one: 68 + 27 = 95 against a printed 94.

Common traps

• Testing only the first-to-second step and never checking the third number
• Splitting the numbers into digits, which the stem explicitly bars
• Accepting a near miss, when 95 against 94 is a miss

No directly related past PYQ was found.