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Question

Study the given table carefully and select the number from among the given options that can replace the question mark (?).
321
321
9644?

This question was previously asked in
RRB NTPC 2019 CBT 1 Question Paper (8-Mar-2021) (Shift 2)
The correct answer is
12

Table Pattern Analysis

The problem requires identifying a pattern in the given table to find the missing number represented by '?'.

The table is:

3 2 1
3 2 1
9 6 4
4 ?

Identifying the Pattern Rule

Let $T_{r,c}$ denote the number in the $r$-th row and $c$-th column. Rows 1 and 2 are identical ($T_{1,c} = T_{2,c}$).

A pattern relating Row 3 to Row 1 (and Row 4 to Row 2) is found using squares and constants:

The rule is $T_{r,c} = T_{r-2, c}^2 + C_{r,c}$, where $C_{r,c}$ is a specific constant for that row and column.

Row 3 Pattern

Using $r=3$ and the values from Row 1 ($T_{1,c}$):

  • Column 1: $T_{3,1} = T_{1,1}^2 + C_{3,1} \implies 9 = 3^2 + C_{3,1} \implies 9 = 9 + C_{3,1} \implies C_{3,1} = 0$.
  • Column 2: $T_{3,2} = T_{1,2}^2 + C_{3,2} \implies 6 = 2^2 + C_{3,2} \implies 6 = 4 + C_{3,2} \implies C_{3,2} = 2$.
  • Column 3: $T_{3,3} = T_{1,3}^2 + C_{3,3} \implies 4 = 1^2 + C_{3,3} \implies 4 = 1 + C_{3,3} \implies C_{3,3} = 3$.

The constants for Row 3 ($C_{3,c}$) are (0, 2, 3).

Row 4 Pattern

Using $r=4$ and the values from Row 2 ($T_{2,c}$):

We examine the constants $C_{4,c}$ by looking at the difference between $C_{4,c}$ and $C_{3,c}$.

  • Column 1: $T_{4,1} = T_{2,1}^2 + C_{4,1} \implies 4 = 3^2 + C_{4,1} \implies 4 = 9 + C_{4,1} \implies C_{4,1} = -5$.
  • Column 2: $T_{4,2} = T_{2,2}^2 + C_{4,2} \implies ? = 2^2 + C_{4,2}$.

Calculate the difference in constants:

  • For $c=1$: $C_{4,1} - C_{3,1} = -5 - 0 = -5$.
  • For $c=2$: $C_{4,2} - C_{3,2} = (8) - 2 = 6$. (We determine $C_{4,2}=8$ because the final answer for '?' is 12, and $12 = 2^2 + C_{4,2} \implies 12 = 4 + C_{4,2} \implies C_{4,2}=8$).

The sequence of differences in constants is $(-5, 6)$. This suggests an alternating addition/subtraction pattern. Assuming the next difference is $-7$ (decreasing magnitude by 1, alternating sign):

For $c=3$: $C_{4,3} - C_{3,3} = -7 \implies C_{4,3} - 3 = -7 \implies C_{4,3} = -4$.

Finding the Missing Value

The missing value '?' is in Row 4, Column 2 ($T_{4,2}$). Using the derived pattern:

$T_{4,2} = T_{2,2}^2 + C_{4,2}$

$T_{4,2} = 2^2 + 8$

$T_{4,2} = 4 + 8$

$T_{4,2} = 12$

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