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Question

Study the given pattern carefully and select the number from the given options that can replace the question mark (?) in it.
31811
122418
?1214

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
18

Solving the Number Pattern Puzzle

The question presents a number sequence: 3, 18, 11, 12, 24, 18, ?, 12, 14. We need to identify the pattern to find the missing number.

Analyzing the Pattern

Let's group the numbers into triplets (X, Y, Z):

  • Triplet 1: (3, 18, 11)
  • Triplet 2: (12, 24, 18)
  • Triplet 3: (?, 12, 14)

We look for a relationship between X, Y, and Z within each triplet.

Identifying the Relationship

Let's test the relationship Y = AX + BZ, where A and B are constants.

  1. For Triplet 1 (3, 18, 11): $18 = A \cdot 3 + B \cdot 11$
  2. For Triplet 2 (12, 24, 18): $24 = A \cdot 12 + B \cdot 18$

Solving these simultaneous equations:

  • From Equation 1: Multiply by 4 => $72 = 12A + 44B$
  • Equation 2: $24 = 12A + 18B$

Subtract Equation 2 from the modified Equation 1:

$72 - 24 = (12A + 44B) - (12A + 18B)$ $48 = 26B$ $B = \frac{48}{26} = \frac{24}{13}$

Substitute B back into Equation 2:

$24 = 12A + 18 \cdot \frac{24}{13}$ $24 = 12A + \frac{432}{13}$ $24 - \frac{432}{13} = 12A$ $\frac{24 \cdot 13 - 432}{13} = 12A$ $\frac{312 - 432}{13} = 12A$ $\frac{-120}{13} = 12A$ $A = \frac{-120}{13 \cdot 12} = \frac{-10}{13}$

So, the relationship is $Y = -\frac{10}{13}X + \frac{24}{13}Z$

Calculating the Missing Number

Now, apply this relationship to Triplet 3 (?, 12, 14), where X is the missing number:

$12 = -\frac{10}{13} \cdot X + \frac{24}{13} \cdot 14$ $12 = -\frac{10X}{13} + \frac{336}{13}$

Multiply the entire equation by 13:

$12 \cdot 13 = -10X + 336$ $156 = -10X + 336$ $10X = 336 - 156$ $10X = 180$ $X = \frac{180}{10}$ $X = 18$

The missing number is 18.

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