6, 11, 21, 36, 56, ? — Aptitude Question
6, 11, 21, 36, 56, ?: Find the next term: 6, 11, 21, 36, 56, ? Answer and worked solution on CodeKairo.
- Topic: Number & Letter Series
- Section: Logical Reasoning
- Difficulty: Medium
- Time target: 50 seconds
Question
Find the next term:
6, 11, 21, 36, 56, ?
Options
- 76
- 81
- 86
- 91
Show the answer and the worked solution
Correct answer: B. 81
Worked solution
- Differences: 5, 10, 15, 20.
- Next = 56 + 25 = 81.
Answer: 81.
The idea
Differences that are multiples of a fixed number make the next step obvious once written down.
Shortcut. The gaps are 5 × 1, 2, 3, 4, so term n = 6 + 5 × (1 + 2 + … + (n − 1)); the sixth term is 6 + 5 × 15 = 81.
Common trap. 76 — repeating the last gap, 20 (56 + 20); the gaps grow by 5, so the next is 25.