Question Details

Given below are two number series. Series I is a missing series while series II is a wrong number series which follows pattern of series I only.

1. 11, P, 181, 350, 639, 1000
2. 242, 251, 255, 280, 329, 450, 619

If 2 is added in successive terms of a series starting with the nearest prime of P, then find the 5th term of this new series
1. 67
2. 65
3. 69
4. 63

Options

A

either 1 or 3

B

either 2 or 3

C

either 2 or 4

D

either 3 or 4

E

either 1 or 2

Show Answer

Correct Answer :

Option A

either 1 or 3

Solution :

The correct answer is either 1 or 3.

Let us analyze the two number series step-by-step.

Step 1: Analyze Series I to find the pattern and the value of P
Series I is given as:
11, P, 181, 350, 639, 1000
Let us find the differences between successive terms towards the end of the series:
Difference between 1000 and 639:
1000-639=361
Note that 361=192.
Difference between 639 and 350:
639-350=289
Note that 289=172.
Difference between 350 and 181:
350-181=169
Note that 169=132.
The differences are the squares of prime numbers in decreasing order: 192, 172, 132.
Continuing this prime pattern backwards, the primes smaller than 13 are 11, 7, 5, etc.
Thus, the preceding differences should be 112=121 and 72=49.
Let us check if this holds for the terms 11, P, and 181:
If the difference between 181 and P is 112=121:
P=181-121=60.
Now let us check the difference between P and 11:
60-11=49, which is indeed 72.
Thus, the pattern is consistent, and P=60.

Step 2: Determine the nearest prime of P
The value of P is 60.
The prime numbers closest to 60 are 59 (which is 60-1) and 61 (which is 60+1).
Both 59 and 61 are at an equal distance of 1 unit from 60. Therefore, the nearest prime of P can be either 59 or 61.

Step 3: Find the 5th term of the new series
The question states that we start with the nearest prime of P and add 2 to successive terms.
This means the difference between successive terms is 2 (an arithmetic progression with common difference d=2).
The formula for the n-th term of an arithmetic progression is:
Tn=a+(n-1)d
Here, we want to find the 5th term (n=5) and the common difference is d=2.
T5=a+(5-1)×2=a+8

Case 1: If the starting prime is 59
First term a=59.
The series is: 59, 61, 63, 65, 67...
The 5th term is:
T5=59+8=67 (This corresponds to statement/option 1).

Case 2: If the starting prime is 61
First term a=61.
The series is: 61, 63, 65, 67, 69...
The 5th term is:
T5=61+8=69 (This corresponds to statement/option 3).

Therefore, the 5th term of the new series is either 1 or 3 (either 67 or 69).

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