The first thing we have to know is that in a deck of 52 cards we have 4 cards with the number 7.
Then the probability is given by the total number of cards with the number 7 over the total of cards in this case 52
[tex]P(card)=\frac{Card}{\text{total}}_{}[/tex][tex]\begin{gathered} P(7)=\frac{4}{52} \\ P(7)=\frac{1}{13} \end{gathered}[/tex]