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Question

Select the number set from among the given options that can replace the question mark (?) in the following series.
4 : 9, 25 : 49, ?, 289 : 361

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper PDF (31-Jan-2021) (Shift 1)
The correct answer is
121 : 169

Pattern Recognition in Number Series

The question asks to find the missing number pair in a given series based on a pattern. The series is: 4 : 9, 25 : 49, ? : ?, 289 : 361.

Analyzing Number Pairs

Let's examine the relationship between the numbers in each pair:

  • First pair: 4 and 9. Note that $4 = 2^2$ and $9 = 3^2$.
  • Second pair: 25 and 49. Note that $25 = 5^2$ and $49 = 7^2$.
  • Fourth pair: 289 and 361. Note that $289 = 17^2$ and $361 = 19^2$.

Identifying the Prime Number Sequence

The bases of these squares form a sequence: 2, 3, 5, 7, ?, ?, 17, 19.

Observing this sequence, we can identify it as the sequence of prime numbers.

Determining the Missing Numbers

The prime numbers between 7 and 17 are 11 and 13.

Therefore, the missing bases in the sequence are 11 and 13.

The missing number pair should be the squares of these prime numbers: $11^2$ and $13^2$.

Calculating the Missing Pair

  • Calculate the square of 11: $11^2 = 121$.
  • Calculate the square of 13: $13^2 = 169$.

The missing number pair is 121 : 169.

Matching with Options

Comparing this result with the given options:

  • Option 1: 729 : 64 (Incorrect)
  • Option 2: 225 : 245 (Incorrect)
  • Option 3: 121 : 169 (Correct)
  • Option 4: 49 : 216 (Incorrect)

The number set that correctly replaces the question mark is 121 : 169.

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