[tex]M=\left \{ x\:\in\:\mathbb{N^{*}}\:| \:x<21\right \}[/tex]
[tex]x \: \in \: \mathbb{{N}^{*}} \: si \: x < 21 = > x \: \in \: \left \{ 1,2,3,...,20 \right \}[/tex]
[tex] = > nr. \: cazuri \: posibile = card M = 20- 1 + 1 = 20[/tex]
[tex]nr. \: cazuri \: favorabile = \left \{ 1,2,3,5,7,11,13,17,19\right \} = 9[/tex]
[tex]P = \frac{nr. \: cazuri \: favorabile}{nr. \: cazuri \: posibile} = \frac{9}{20} [/tex]
