Question

A prime number is called a super-prime if doubling it and then subtracting 1 results in another prime number, then the number of Superprimes less than 15 is ______.

• A. 2
• B. 3
• C. 4
• D. 8

Answer 1

The correct option is B 3

Clearly,
$2\left(2\right)-1=3$, which is prime
$2\left(3\right)-1=5$, which is prime
$2\left(7\right)-1=13$, which is prime

Hence, $2,3,7$ are superprimes.

The number of Superprimes less than $15$ are $3$.
Option B is correct.