13 of the 18 players on the varsity soccer team are seniors. What percent of the players on the team are not seniors?

there are 18 players, 13 are seniors, that means 18 - 13 are not seniors, namely 5.

now if we take 18 to be the 100%, what is 5 off of it in percentage?

[tex]\bf \begin{array}{ccll} amount&\%\\ \text{\textemdash\textemdash\textemdash}&\text{\textemdash\textemdash\textemdash}\\ 18&100\\ 5&p \end{array}\implies \cfrac{18}{5}=\cfrac{100}{p}\implies p=\cfrac{5\cdot 100}{18}[/tex]