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Question

What will come in the place of question mark (?) in the following number series?

3, 6, 18, ?, 630, 6930

This question was previously asked in
RBI Assistant Prelims Memory Based Paper (27 March 2022) (Shift 2)
The correct answer is

90

Number Series Pattern Analysis

The question asks us to find the missing number in the given number series: 3, 6, 18, ?, 630, 6930.

We need to identify the logical relationship or pattern between the consecutive numbers in the series to determine the value that should replace the question mark.

Finding the Relationship Between Terms

Let's examine the relationship between the first few terms:

  • From 3 to 6: The number is multiplied by 2. (

    3 \times 2 = 6

    )
  • From 6 to 18: The number is multiplied by 3. (

    6 \times 3 = 18

    )

We observe that the multiplier is increasing. Let's hypothesize that the multipliers are sequential prime numbers.

Step-by-Step Solution Using Prime Multipliers

The sequence of multipliers starts with 2, then 3. The next prime numbers are 5, 7, 11, and so on. Let's test this hypothesis:

Term Multiplier Next Term Calculation
3

2

(1st Prime)
6

3 \times 2 = 6

6

3

(2nd Prime)
18

6 \times 3 = 18

18

5

(3rd Prime)
?

18 \times 5 = 90

? (Result is 90)

7

(4th Prime)
630

90 \times 7 = 630

630

11

(5th Prime)
6930

630 \times 11 = 6930


The pattern holds true. The series is generated by multiplying the current term by consecutive prime numbers starting from 2.

Identifying the Missing Number

Based on the identified pattern of multiplying by consecutive prime numbers (2, 3, 5, 7, 11), the missing number is calculated as follows:

  • The term before the question mark is 18.
  • The multiplier needed is the next prime number after 3, which is 5.
  • Calculation:

    18 \times 5 = 90

To confirm, the next step should be multiplying 90 by the next prime number, 7:

  • Calculation:

    90 \times 7 = 630

This matches the subsequent term in the series. Therefore, the missing number is 90.

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