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Question

In the following question, select the missing number from the given series.

4, 13, 41, 126, 382, ?

This question was previously asked in
SSC Stenographer 2017 Previous Year Paper (14-Sep-2017) (Shift 1)
The correct answer is

1151

Analyzing the Number Series Pattern

This question asks us to find the missing number in the given series: 4, 13, 41, 126, 382, ?

To find the missing number, we need to identify the pattern or rule that connects the terms in the series. Let's look at the relationship between consecutive numbers:

  • From 4 to 13: \(13 - 4 = 9\). What else could it be? \(4 \times 2 + 5 = 13\)? \(4 \times 3 + 1 = 13\)? Let's test the multiplication pattern.
  • From 13 to 41: \(41 - 13 = 28\). If the pattern is \(\times 3 + \text{something}\), let's check: \(13 \times 3 = 39\). \(39 + 2 = 41\). This looks promising. Let's see if the 'something' follows a pattern.
  • From 41 to 126: \(126 - 41 = 85\). Following the \(\times 3 + \text{something}\) idea: \(41 \times 3 = 123\). \(123 + 3 = 126\). The added number increased from 2 to 3.
  • From 126 to 382: \(382 - 126 = 256\). Following the \(\times 3 + \text{something}\) idea: \(126 \times 3 = 378\). \(378 + 4 = 382\). The added number increased from 3 to 4.

The pattern appears to be: Multiply the previous term by 3 and add a number that increases by 1 each time, starting from 1.

Let's verify this pattern for the initial terms:

  • Term 1: 4
  • Term 2: \(4 \times 3 + 1 = 12 + 1 = 13\) (Correct)
  • Term 3: \(13 \times 3 + 2 = 39 + 2 = 41\) (Correct)
  • Term 4: \(41 \times 3 + 3 = 123 + 3 = 126\) (Correct)
  • Term 5: \(126 \times 3 + 4 = 378 + 4 = 382\) (Correct)

The pattern is consistently \(\text{Previous Term} \times 3 + n\), where \(n\) is 1 for the second term, 2 for the third, 3 for the fourth, and 4 for the fifth. For the missing sixth term, \(n\) should be 5.

Calculating the Missing Number

Based on the identified pattern, the next term in the series is calculated by taking the last known term (382), multiplying it by 3, and adding 5.

Missing Term \(= 382 \times 3 + 5\)

First, calculate \(382 \times 3\):

\(382 \times 3 = 1146\)

Now, add 5 to the result:

\(1146 + 5 = 1151\)

So, the missing number in the series is 1151.

Verification

The series with the calculated missing number is 4, 13, 41, 126, 382, 1151.

Let's confirm the steps:

Term Calculation Value
1 Given 4
2 \(4 \times 3 + 1\) 13
3 \(13 \times 3 + 2\) 41
4 \(41 \times 3 + 3\) 126
5 \(126 \times 3 + 4\) 382
6 \(382 \times 3 + 5\) 1151

The calculation confirms that 1151 follows the established pattern.

Conclusion

The pattern in the series is multiplying the previous term by 3 and adding an incrementing number (1, 2, 3, 4, 5, ...). Applying this pattern to the last term 382 gives \(382 \times 3 + 5 = 1151\). Therefore, the missing number is 1151.

Revision Table: Number Series Logic

Concept Description Example Pattern
Arithmetic Series A constant difference between consecutive terms. 2, 5, 8, 11... (difference is 3)
Geometric Series A constant ratio between consecutive terms. 3, 6, 12, 24... (ratio is 2)
Mixed Series Combines arithmetic and geometric operations, or uses a pattern on the operation/increment. This problem's pattern (\( \times 3 + n \)) is a type of mixed series.
Difference Series Finding patterns in the differences between consecutive terms. 1, 4, 9, 16... (differences are 3, 5, 7... which is an arithmetic series)
Step-by-Step Pattern Analysis Examining the relationship between each pair of consecutive numbers to deduce the rule. As done in the solution: check difference, ratio, and combinations like \(\times a + b\) or \(+ a \times b\).

Additional Information: Solving Number Series Problems

Solving number series problems often involves looking for common mathematical operations connecting terms. Here are some strategies:

  • Look for differences: Calculate the difference between consecutive terms. Is there a pattern in the differences?
  • Look for ratios: Calculate the ratio between consecutive terms (divide a term by the previous one). Is there a constant ratio?
  • Consider squares and cubes: The series might involve squaring or cubing the term number or the previous term, possibly with an addition or subtraction.
  • Combine operations: The pattern might involve a combination of multiplication/division and addition/subtraction, as seen in this problem (\( \times 3 + n \)).
  • Look at alternating patterns: Sometimes, the pattern applies to alternate terms, or there are two interleaved series.
  • Check for prime numbers or other specific sequences: The series might be composed of prime numbers, Fibonacci numbers, etc.

Systematically testing these possibilities is key to finding the underlying logic of a number series.

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