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Question

In the following series, one number is missing as shown by the question mark (?). Select the missing number form the given options.

8, 12, 16, 20, ?

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is

24

Analyzing the Number Series Pattern

The question asks us to find the missing number in the series: 8, 12, 16, 20, ?.

To find the missing number in a series, we need to identify the pattern or rule that connects the numbers.

Identifying the Pattern in the Series

Let's look at the difference between consecutive terms in the given number series:

  • Difference between the second and first term: $$12 - 8 = 4$$
  • Difference between the third and second term: $$16 - 12 = 4$$
  • Difference between the fourth and third term: $$20 - 16 = 4$$

We can observe that the difference between each consecutive term is a constant value of 4. This indicates that the series is an arithmetic progression where each term is obtained by adding 4 to the previous term.

Finding the Missing Number

Following the identified pattern, the next term in the series (the missing number) will be obtained by adding 4 to the last given term, which is 20.

Missing number $$= \text{Last term} + \text{Common difference}$$

Missing number $$= 20 + 4 = 24$$

Therefore, the missing number in the series 8, 12, 16, 20, ? is 24.

Checking the Options

Let's compare our calculated missing number with the given options:

  1. 24
  2. 22
  3. 32
  4. 26

Our calculated missing number, 24, matches option 1.

Conclusion

The number series follows the pattern of adding 4 to the previous term. By applying this pattern, the missing number is found to be 24.

Revision Table: Number Series

Term Number Value Pattern Applied
1st 8 Starting term
2nd 12 8 + 4
3rd 16 12 + 4
4th 20 16 + 4
5th (Missing) 24 20 + 4

Additional Information: Arithmetic Progressions

A number series where the difference between consecutive terms is constant is called an arithmetic progression (AP). This constant difference is known as the common difference ($$d$$).

The formula for the n-th term ($$a_n$$) of an arithmetic progression is:

$$a_n = a_1 + (n-1)d$$

where:

  • $$a_n$$ is the n-th term
  • $$a_1$$ is the first term
  • $$n$$ is the term number
  • $$d$$ is the common difference

In the given series: 8, 12, 16, 20, ?

  • The first term ($$a_1$$) is 8.
  • The common difference ($$d$$) is 4.
  • We are looking for the 5th term ($$a_5$$).

Using the formula:

$$a_5 = a_1 + (5-1)d$$

$$a_5 = 8 + (4)4$$

$$a_5 = 8 + 16$$

$$a_5 = 24$$

This confirms our earlier result obtained by simply adding the common difference to the last term.

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