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Number series questions

Reasoning Updated 11 Oct 2026

These number series questions with answers help you practise one of the most common reasoning topics in SSC, bank, railway and placement exams. The page has number series at easy, medium and hard levels, missing number and wrong number questions, alphabet series and mixed letter number series.

Every answer has been worked out and checked, and each explanation shows the pattern in one to three steps. Try to spot the rule before you look at the options, then confirm it on every term of the series, not just the last two.

Answered 0 / 33Right 0Wrong 0

Number series questions: easy

  1. Q1. What comes next in the series: 3, 7, 11, 15, ?

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    Answer: D. 19
    Each term increases by 4: 15 + 4 = 19.

  2. Q2. What comes next in the series: 5, 10, 20, 40, ?

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    Answer: B. 80
    Each term is double the previous one: 40 x 2 = 80.

  3. Q3. What comes next in the series: 1, 4, 9, 16, 25, ?

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    Answer: A. 36
    These are squares of 1, 2, 3, 4, 5, so the next is 6^2 = 36.

  4. Q4. What comes next in the series: 100, 91, 82, 73, ?

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    Answer: C. 64
    Each term decreases by 9: 73 - 9 = 64.

  5. Q5. What comes next in the series: 2, 6, 18, 54, ?

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    Answer: B. 162
    Each term is multiplied by 3: 54 x 3 = 162.

  6. Q6. What comes next in the series: 1, 8, 27, 64, ?

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    Answer: D. 125
    These are cubes of 1, 2, 3, 4, so the next is 5^3 = 125.

Number series questions: medium

  1. Q7. What comes next in the series: 1, 2, 4, 7, 11, ?

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    Answer: C. 16
    The differences are +1, +2, +3, +4, so the next difference is +5: 11 + 5 = 16.

  2. Q8. What comes next in the series: 2, 5, 11, 23, 47, ?

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    Answer: A. 95
    Each term is (previous x 2) + 1: 47 x 2 + 1 = 95.

  3. Q9. What comes next in the series: 3, 5, 9, 17, 33, ?

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    Answer: C. 65
    Each term is (previous x 2) - 1: 33 x 2 - 1 = 65.

  4. Q10. What comes next in the series: 1, 1, 2, 3, 5, 8, ?

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    Answer: B. 13
    Each term is the sum of the two before it: 5 + 8 = 13.

  5. Q11. What comes next in the series: 2, 3, 5, 7, 11, 13, ?

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    Answer: A. 17
    These are consecutive prime numbers; the prime after 13 is 17.

  6. Q12. What comes next in the series: 4, 6, 12, 14, 28, 30, ?

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    Answer: D. 60
    The pattern alternates +2 and x2: 4+2=6, 6x2=12, 12+2=14, 14x2=28, 28+2=30, so 30 x 2 = 60.

Number series questions: hard

  1. Q13. What comes next in the series: 6, 11, 21, 36, 56, ?

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    Answer: B. 81
    The differences are 5, 10, 15, 20, so the next difference is 25: 56 + 25 = 81.

  2. Q14. What comes next in the series: 2, 12, 36, 80, 150, ?

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    Answer: C. 252
    Each term is n^3 + n^2: 2, 12, 36, 80, 150, so the next is 6^3 + 6^2 = 216 + 36 = 252.

  3. Q15. What comes next in the series: 7, 8, 12, 21, 37, ?

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    Answer: A. 62
    The differences are 1, 4, 9, 16 (squares), so the next difference is 25: 37 + 25 = 62.

  4. Q16. What comes next in the series: 120, 60, 20, 5, ?

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    Answer: D. 1
    The terms are divided by 2, 3, 4 and then 5: 5 / 5 = 1.

  5. Q17. What comes next in the series: 1, 2, 6, 24, 120, ?

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    Answer: D. 720
    Each term is multiplied by 2, 3, 4, 5 and then 6: 120 x 6 = 720.

  6. Q18. What comes next in the series: 1, 3, 7, 15, ?

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    Answer: A. 31
    Each term is (previous x 2) + 1, which gives 2^n - 1: 15 x 2 + 1 = 31.

Missing number and wrong number questions

  1. Q19. Find the wrong number in the series: 2, 4, 8, 16, 30, 64

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    Answer: C. 30
    Each term should double: 16 x 2 = 32, not 30.

  2. Q20. Find the wrong number in the series: 1, 4, 9, 15, 25, 36

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    Answer: B. 15
    The series is the squares 1, 4, 9, 16, 25, 36, so 15 should be 16.

  3. Q21. Find the wrong number in the series: 5, 10, 15, 21, 25, 30

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    Answer: C. 21
    The series counts in fives (multiples of 5), so 21 should be 20.

  4. Q22. Find the missing number: 3, ?, 12, 24, 48

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    Answer: A. 6
    Each term doubles: 3 x 2 = 6 and 6 x 2 = 12.

  5. Q23. Find the missing number: 11, 13, 17, 19, 23, ?

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    Answer: B. 29
    These are consecutive primes; after 23, the numbers 24 to 28 are not prime, so the next prime is 29.

  6. Q24. Find the missing number: 2, 5, 11, ?, 32, 47

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    Answer: D. 20
    The differences are 3, 6, 9, 12, 15. So 11 + 9 = 20, and 20 + 12 = 32 checks out.

Alphabet series reasoning questions

  1. Q25. What comes next in the letter series: A, C, E, G, ?

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    Answer: A. I
    Each letter skips one: A, C, E, G, so the next is I (G + 2).

  2. Q26. What comes next in the letter series: Z, X, V, T, ?

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    Answer: D. R
    The letters move back by 2: Z(26), X(24), V(22), T(20), so the next is R(18).

  3. Q27. What comes next in the letter series: B, E, H, K, ?

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    Answer: C. N
    Each letter moves 3 places forward: B(2), E(5), H(8), K(11), so the next is N(14).

  4. Q28. What comes next in the letter series: AZ, BY, CX, ?

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    Answer: B. DW
    The first letter moves forward from A and the second moves back from Z: AZ, BY, CX, so the next is DW.

  5. Q29. What comes next in the letter series: A, B, D, G, K, ?

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    Answer: D. P
    The gaps grow by one: A(1), B(2), D(4), G(7), K(11), so the next is 11 + 5 = 16, which is P.

  6. Q30. What comes next in the letter series: ACE, BDF, CEG, ?

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    Answer: B. DFH
    Each group is three alternate letters starting one place later: ACE, BDF, CEG, so the next is DFH.

  7. Q31. What comes next in the letter series: Z, U, P, K, ?

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    Answer: A. F
    The letters move back by 5: Z(26), U(21), P(16), K(11), so the next is F(6).

Letter and number mixed series

  1. Q32. What comes next in the series: A1, C3, E5, G7, ?

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    Answer: C. I9
    Each term is a letter with its position number: A1, C3, E5, G7, so the next is I9.

  2. Q33. What comes next in the series: 2B, 4D, 6F, 8H, ?

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    Answer: A. 10J
    Each term is an even number with the letter at that position: 2B, 4D, 6F, 8H, so the next is 10J.

How to prepare

To crack a number series, first check the differences between terms. If the differences are equal it is an arithmetic series; if they rise steadily, look at the second differences or at squares (1, 4, 9, 16) and cubes. If the numbers grow very fast, divide each term by the one before to find a multiplication rule such as x2, x3 or x2 + 1. When nothing fits, check for two series alternating with each other, or for primes and the Fibonacci pattern.

For alphabet series, convert letters to their positions (A = 1, Z = 26) and treat the series as numbers. Learn the positions of E(5), J(10), O(15), T(20) and Y(25) as anchors, and the opposite pairs that add to 27. In missing and wrong number questions, find the rule from the terms you trust and test each term against it.

Study tips

  • Memorise squares up to 30 and cubes up to 15, since many series are built on them.
  • Write the differences under the series every time; the pattern usually appears in the first or second row.
  • Use EJOTY (5, 10, 15, 20, 25) to find any letter's position in seconds.

More practice questions

Questions people ask

How do I solve number series questions quickly?

Start with the differences between terms, then check ratios if the numbers grow fast. If neither works, look for squares, cubes, primes, Fibonacci or two alternating series. Always verify the rule on every term before choosing an option.

What is a missing number reasoning question?

It is a series with one term replaced by a question mark, which may be in the middle rather than at the end. Find the rule from the other terms and use it to fill the gap, then check that the term after the gap still fits.

How are alphabet series questions solved?

Convert each letter to its position number (A = 1 to Z = 26) and look for a fixed jump, a growing jump or a reverse pattern. After Z the count starts again at A.

Which exams ask series reasoning questions?

Number and letter series appear in SSC CGL, CHSL and MTS, bank PO and clerk exams, railway exams, state exams and campus placement aptitude tests. They usually carry 2 to 5 questions per paper.

Every question on this page is written and fact-checked by the GK Quizzes team. Spotted a mistake? Tell us and we will fix it. How we make our questions.