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Question

What should come in place of the question mark (?) in the given series?
3, −2, −12, −32, −72, ?

This question was previously asked in
RRB NTPC 2024 CBT 1 Question Paper (28-Aug-2025) (Shift 3)
The correct answer is
−152

Series Differences

The given number series is: 3, −2, −12, −32, −72, ?

Calculate the differences between consecutive terms to identify a pattern.

Term Pair Difference ($a_{n+1} - a_n$)
3 and −2 $-2 - 3 = -5$
−2 and −12 $-12 - (-2) = -10$
−12 and −32 $-32 - (-12) = -20$
−32 and −72 $-72 - (-32) = -40$

Difference Pattern

The calculated differences are: $-5, -10, -20, -40$.

Notice that each difference is twice the previous difference:

  • $-10 = -5 \times 2$
  • $-20 = -10 \times 2$
  • $-40 = -20 \times 2$

Following this pattern, the next difference is $-40 \times 2 = -80$.

Next Term Calculation

To find the missing term (the next term in the series), add this next difference ($-80$) to the last term of the series ($-72$).

Missing Term = Last Term + Next Difference

Missing Term = $-72 + (-80)$

Missing Term = $-72 - 80$

Missing Term = $-152$

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