Here are the sides of the 20-sided die:

Side   prime?    ≧15?    Prime and ≧15?
----------------------------------------   
1,       no,     no            no
2,      yes,     no            no
3,      yes,     no            no
4,       no,     no            no
5,      yes,     no            no
6,       no,     no            no
7,      yes,     no            no
8,       no,     no            no
9,       no,     no            no
10,      no,     no            no
11,     yes,     no            no
12,      no,     no            no
13,     yes,     no            no
14,      no,     no            no
15,      no,    yes            no
16,      no,    yes            no
17,     yes,    yes           yes
18,      no,    yes            no
19,     yes,    yes           yes
20       no,    yes            no

Only 17 and 19 are prime and also ≧ 15.

Answer: The probability is 2 out of 20 or  which reduces to .

Edwin