9, 27, 31, 155, 161, ? — Aptitude Question
9, 27, 31, 155, 161, ?: Find the next term: 9, 27, 31, 155, 161, ? Answer and worked solution on CodeKairo.
- Topic: Number & Letter Series
- Section: Logical Reasoning
- Difficulty: Hard
- Time target: 120 seconds
Question
Find the next term:
9, 27, 31, 155, 161, ?
Options
- 805
- 966
- 1,127
- 1,288
Show the answer and the worked solution
Correct answer: C. 1,127
Worked solution
- 9 × 3 = 27; 27 + 4 = 31; 31 × 5 = 155; 155 + 6 = 161.
- The next operation is × 7: 161 × 7 = 1,127.
Answer: 1,127.
The idea
Alternating operations are common in the harder papers. Look at each step separately rather than trying to find one rule for the whole series.
Shortcut. The operands 3, 4, 5, 6 simply count up, so the sixth step is × 7. Units digit: 161 × 7 ends in 7, and only 1,127 does.
Common trap. 966 — multiplying 161 by 6, the last number used; 6 was an addition, and the operand has moved on to 7.