Series Completion— Concepts, Formulas & Shortcuts
- Take consecutive differences first; a constant difference means an arithmetic series.
- If differences themselves form a pattern (2, 4, 6, 8), the series is quadratic.
- Constant ratios indicate a geometric series.
- For alphanumeric series, split the letters and numbers into two separate sequences.
- Alternating series follow one rule on odd positions and another on even positions.
- Always verify your rule against every given term before answering.
Series Completion Practice Questions with Answers
Attempt each question first, then open the explanation. All 10 questions below are free to read and require no signup.
Q1.Complete the series: 4, 8, 12, 16, ?
Easy- A18
- B20
- C22
- D24
Series Completion question 1 of 10+Show Answer & Explanation
Answer: B. 20
Explanation: A constant difference of 4 gives 16 + 4 = 20.
Q2.Complete the series: 3, 9, 27, 81, ?
Easy- A162
- B216
- C243
- D324
Series Completion question 2 of 10+Show Answer & Explanation
Answer: C. 243
Explanation: Each term is multiplied by 3: 81 × 3 = 243.
Q3.Complete the series: 2, 6, 12, 20, 30, ?
Moderate- A36
- B40
- C42
- D48
Series Completion question 3 of 10+Show Answer & Explanation
Answer: C. 42
Explanation: Differences run 4, 6, 8, 10, so the next is 12 → 30 + 12 = 42.
Q4.Complete the series: A1, B2, C3, ?
Easy- AD4
- BE4
- CD5
- DE5
Series Completion question 4 of 10+Show Answer & Explanation
Answer: A. D4
Explanation: Letters advance A, B, C, D while numbers advance 1, 2, 3, 4.
Q5.Complete the series: 1, 4, 9, 16, 25, ?
Easy- A30
- B36
- C42
- D49
Series Completion question 5 of 10+Show Answer & Explanation
Answer: B. 36
Explanation: These are perfect squares, so the next is 6² = 36.
Q6.Complete the series: Z1, Y2, X3, ?
Moderate- AW4
- BV4
- CW5
- DU4
Series Completion question 6 of 10+Show Answer & Explanation
Answer: A. W4
Explanation: Letters retreat Z, Y, X, W while numbers advance 1, 2, 3, 4.
Q7.Complete the series: 5, 11, 23, 47, ?
Moderate- A85
- B91
- C94
- D95
Series Completion question 7 of 10+Show Answer & Explanation
Answer: D. 95
Explanation: Each term is doubled and increased by 1: 47 × 2 + 1 = 95.
Q8.Complete the series: 2, 3, 5, 8, 12, ?
Moderate- A15
- B16
- C17
- D18
Series Completion question 8 of 10+Show Answer & Explanation
Answer: C. 17
Explanation: Differences are 1, 2, 3, 4, so the next is 5 → 12 + 5 = 17.
Q9.Complete the series: 100, 50, 25, ?
Easy- A10
- B12
- C12.5
- D15
Series Completion question 9 of 10+Show Answer & Explanation
Answer: C. 12.5
Explanation: Each term is halved: 25 ÷ 2 = 12.5.
Q10.Complete the series: 1, 2, 4, 7, 11, 16, ?
Moderate- A20
- B21
- C22
- D24
Series Completion question 10 of 10+Show Answer & Explanation
Answer: C. 22
Explanation: Differences are 1, 2, 3, 4, 5, so the next is 6 → 16 + 6 = 22.
Series Completion — Frequently Asked Questions
How do I handle a mixed letter-and-number series?+
Separate the two streams. Analyse the letters as alphabet positions and the numbers independently, then recombine to produce the missing term.
What if the differences look random?+
Split the series into odd-position and even-position terms. Wildly swinging differences almost always signal two interleaved sequences.
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