Which one of the following numbers in the sequence of numbers given below is different from the rest? 22, 33, 99, 121, 279, 594

The answer is C) 279. The series is built on divisibility by 11. Check each number: 22 = 2 × 11, 33 = 3 × 11, 99 = 9 × 11, 121 = 11 × 11, and 594 = 54 × 11. Only 279 fails: 279 = 9 × 31, so it is not a multiple of 11.

The quick test is the alternating-sum rule for 11. For 279, 9 − 7 + 2 = 4, which is not divisible by 11. For 594, 4 − 9 + 5 = 0, so it passes.

• A) 33 is 3 × 11, so it belongs to the group.
• B) 121 is 11², so it belongs to the group.
• D) 594 is 54 × 11, so it belongs to the group.

This is a number-series and divisibility question, a staple of the UPPSC reasoning and aptitude section. Recognising prime factors quickly is a skill that also helps in the arithmetic of Mains data questions.

Trap: Aspirants try digit sums. The digit sums are 4, 6, 18, 4, 18, 18, which point to 22, 33 or 121 as the odd one out. Always test prime factors before using digit patterns.