Among employees of a certain firm, $70\%$ know C$/$C$++,60\%$ know Fortran, and $50\%$ know both languages.

An employee is randomly chosen from the firm. What is the probability the employee

$($a$)$ does not know Fortran?

$($b$)$ does not know Fortran and does not know C$/$C$++?$

$($c$)$ knows C$/$C$++$ but not Fortran?