80% of 300 = (? - 30) × 8

$frac{{80 imes 300}}{{100}} = left( {? - 30} ight) imes 8$

$Rightarrow 80 imes 3 = left( {? - 30} ight) imes 8$

$Rightarrow ? - 30 = frac{{80 imes 3}}{8} = 30$

$Rightarrow ? = 30 + 30 = 60$