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
Correct Answer :
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:
Note that .
Difference between 639 and 350:
Note that .
Difference between 350 and 181:
Note that .
The differences are the squares of prime numbers in decreasing order: , , .
Continuing this prime pattern backwards, the primes smaller than 13 are 11, 7, 5, etc.
Thus, the preceding differences should be and .
Let us check if this holds for the terms 11, P, and 181:
If the difference between 181 and P is :
.
Now let us check the difference between P and 11:
, which is indeed .
Thus, the pattern is consistent, and .
Step 2: Determine the nearest prime of P
The value of P is 60.
The prime numbers closest to 60 are 59 (which is ) and 61 (which is ).
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 ).
The formula for the n-th term of an arithmetic progression is:
Here, we want to find the 5th term () and the common difference is .
Case 1: If the starting prime is 59
First term .
The series is: 59, 61, 63, 65, 67...
The 5th term is:
(This corresponds to statement/option 1).
Case 2: If the starting prime is 61
First term .
The series is: 61, 63, 65, 67, 69...
The 5th term is:
(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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