The smallest value of natural number n, for which 2 n+1 is not a prime number, is · ABC26GN0124
Subject: General Aptitude · Chapter: Number System
The smallest value of natural number $n$, for which $2 n+1$ is not a prime number, is
(a)3
(b)4
(c)5
(d)None of these
Answer
Answer (as printed): B
Explanation
Putting $n=1,2,3,4$ respectively in ( $2 n+1$ ) we get: $(2 \times 1+1)=3,(2 \times 2+1)=5,(2 \times 3+1)=7$ and $(2 \times$ $4+1)=9$, where 3, 5, 7 are prime numbers. ∴ The smallest value of $n$ for which $(2 n+1)$ is not prime, is $n=4$.
Explanation as extracted from the printed page; notation may be imperfect.