A prime number is called a jillyprime when doubling it and adding 1 results in another prime. How many numbers lass than 15 are jillyprimes? (Note that 1 is not a prime.)

2 x 2 + 1 = 5

3 x 2 + 1 = 7

5 x 2 + 1 = 11

7 x 2 + 1 = 15

11 x 2 + 1 = 23

13 x 2 + 1 = 27

So,there are 4 jillyprimes which are less than 15 : 2 ; 3 ; 5 ; 11