120, 99, 80, 63, 48, ? — Aptitude Question
120, 99, 80, 63, 48, ?: Find the next term: 120, 99, 80, 63, 48, ? Answer and worked solution on CodeKairo.
- Topic: Number & Letter Series
- Section: Logical Reasoning
- Difficulty: Medium
- Time target: 75 seconds
Question
Find the next term:
120, 99, 80, 63, 48, ?
Options
- 33
- 35
- 37
- 39
Show the answer and the worked solution
Correct answer: B. 35
Worked solution
- Differences: −21, −19, −17, −15 — shrinking by 2.
- Next = 48 − 13 = 35.
- The terms are n² − 1 for n = 11, 10, 9, 8, 7, 6. ✓
Answer: 35.
The idea
A falling series with shrinking gaps is still a quadratic, just read backwards. Recognising n² − 1 confirms it.
Shortcut. Each term is two numbers two apart multiplied: 10 × 12, 9 × 11, 8 × 10, 7 × 9, 6 × 8, so the next is 5 × 7 = 35.
Common trap. 33 — subtracting the last gap, 15, again (48 − 15); the gaps shrink by 2, so the next drop is 13.