Reasoning

Series Completion: Practice Questions with Answers

17 practice questions on Series Completion, with answers; most also have an explanation or a worked solution. Try each one before you open its answer.

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1. Q.1

   Find the next number in the series: 0, 1, 8, 27, 64, ?

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   Answer: 125

   Solution:

   1. These are perfect cubes: 0³=0, 1³=1, 2³=8, 3³=27, 4³=64.
   2. The next cube is 5³ = 125.

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2. Q.2

   Find the missing number in the series: 1, 4, 2, 8, 3, 12, 4, ?

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   Answer: 16

   Solution:

   1. This looks irregular as one sequence, so split it into odd-position and even-position terms.
   2. Odd positions (1st, 3rd, 5th, 7th): 1, 2, 3, 4 — increasing by 1 each time.
   3. Even positions (2nd, 4th, 6th, 8th): 4, 8, 12, ? — increasing by 4 each time, so the next (8th) term is 16.

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3. Q.3

   Find the next number in the series: 2, 3, 5, 7, 11, ?

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   Answer: 13

   Solution:

   1. These are the prime numbers in order: 2, 3, 5, 7, 11.
   2. The next prime number is 13.

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4. Q.4

   Find the next number in the series: 3, 6, 4, 8, 6, 12, 10, ?

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   Answer: 20

   Solution:

   1. A single alternating ±amount doesn't fit. Try a two-operation cycle instead: 3→6 is ×2, 6→4 is −2, 4→8 is ×2, 8→6 is −2, 6→12 is ×2, 12→10 is −2.
   2. The cycle "×2, then −2" repeats consistently. The last operation shown was −2 (12→10), so the next is ×2.
   3. Next term = 10 × 2 = 20.

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5. Q.5

   Find the next number in the series: 2, 5, 10, 17, 26, ?

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   Answer: 37

   Solution:

   1. Differences between consecutive terms: 5−2=3, 10−5=5, 17−10=7, 26−17=9.
   2. The differences themselves are 3, 5, 7, 9 — increasing by 2 each time, so the next difference is 11.
   3. Next term = 26 + 11 = 37. (This series is actually n² + 1 for n = 1, 2, 3, …)

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6. Q.6

   Find the next number in the series: 3, 6, 12, 24, 48, ?

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   Answer: 96

   Solution:

   1. Each term is double the previous one: 6/3=2, 12/6=2, 24/12=2, 48/24=2.
   2. Next term = 48 × 2 = 96.

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7. Q.7

   Find the next number in the series: 5, 8, 4, 7, 3, 6, 2, ?

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   Answer: 5

   Solution:

   1. Check the pattern of operations rather than a single constant difference: 5→8 is +3, 8→4 is −4, 4→7 is +3, 7→3 is −4, 3→6 is +3, 6→2 is −4.
   2. The operations alternate +3, −4, +3, −4, … and the last one shown was −4 (6→2), so the next one is +3.
   3. Next term = 2 + 3 = 5.

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8. Q.8

   Find the next number in the series: 4, 8, 5, 10, 7, 14, 11, ?

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   Answer: 22. Two-operation cycle "×2, then −3": 4→8→5→10→7→14→11→22.

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9. Q.9

   Find the missing number in the series: 2, 3, 4, 6, 6, 9, 8, ?

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   Answer: 12 (odd positions +2 each: 2,4,6,8; even positions +3 each: 3,6,9,12)

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10. Q.10

    Find the missing number in the series: 2, 3, 5, 7, ?, 13

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    Answer: 11 (the prime numbers in order)

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11. Q.11

    Find the next number in the series: 1, 8, 27, 64, 125, ?

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    Answer: 216 (= 6³)

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12. Q.12

    Find the next number in the series: 3, 7, 13, 21, 31, ?

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    Answer: 43 (differences 4, 6, 8, 10, 12)

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13. Q.13

    Find the next number in the series: 2, 6, 18, 54, ?

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    Answer: 162 (each term × 3)

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14. Q.14

    Find the next number in the series: 3, 6, 11, 18, 27, ?

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    Answer: 38. Differences are 3, 5, 7, 9, 11 (this series is n² + 2).

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15. Q.15

    Find the next term: AZ, BY, CX, DW, ?

    1. A EV
    2. B EU
    3. C FV
    4. D EW
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    Correct answer: (A) EV

    First letters go forward (A,B,C,D->E), second letters go backward (Z,Y,X,W->V): EV.

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16. Q.16

    Find the next letter: AA, CC, EE, GG, ?

    1. A II
    2. B HH
    3. C JJ
    4. D IK
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    Correct answer: (A) II

    Each pair of repeated letters skips forward 2 places in the alphabet (A, C, E, G, I, ...). So the next letter is II.

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17. Q.17

    Find the next term: ZA, YB, XC, WD, ?

    1. A VE
    2. B VF
    3. C UE
    4. D VD
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    Correct answer: (A) VE

    First letters go backward (Z,Y,X,W->V), second letters go forward (A,B,C,D->E): VE.

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