The half life period of a 1st order reaction is 60 minutes. What percentage will be left over after 240 minutes ? 

${\text{t}}_{\text{1/2}}\text{=60\hspace{0.17em}min}$
The no.of half lives = 240 / 60 = 4 (n = 4)
The amount which is left, after four half lives is; 
$\frac{{\left[\text{A}\right]}_{\text{O}}}{\left[\text{A}\right]}\text{=}\frac{{\left[\text{A}\right]}_{\text{O}}}{\text{16}}\text{=\hspace{0.17em}0.0625\hspace{0.17em}}$ of  $A_{0 }$ or 6.25%