In a set of lottery Tickets 7 carry prizes and 25 are blank. If three tickets are drawn then the probability to get a prize is 

a set of lottery 7prizes & 25 black

$\mathrm{S}={}^{32}{\mathrm{C}}_3$

E=to get prize $\Rightarrow \overline{\mathrm{E}}=$ not getting prize

$\mathrm{n}\left(\overline{\mathrm{E}}\right)={}^{25}{\mathrm{C}}_3 \Rightarrow \mathrm{P}\left(\overline{\mathrm{E}}\right)=\frac{{}^{25}{\mathrm{C}}_3}{{}^{32}\mathrm{C}_3} \Rightarrow \mathrm{P}\left(\mathrm{E}\right)=1-\frac{{}^{25}{\mathrm{C}}_3}{{}^{32}\mathrm{C}_3}$