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RRB NTPC Mains or Stage 2 Model Paper 10 with solutions

Question : 3 of 120

Marks: +1, -0

121 ? 81 64 49 36 25

Solution:

Concept:

The given series is a descending sequence of perfect squares.

Explanation:

Observe the known terms:

121 = 11^2

81 = 9^2

64 = 8^2

49 = 7^2

36 = 6^2

25 = 5^2

The squares decrease by

1

in the base number each time.

So the missing term between

11^2

and

9^2

must be

10^2

10^2 = 100

Thus the complete series is:

121,\ 100,\ 81,\ 64,\ 49,\ 36,\ 25

Answer:

The missing number is

100

, which matches option D.

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