The average marks of boys in a class is 52 and that of girls is 42. The average marks of boys and girls combined is 50. The percentage of boys in the class is

Take  $n_1+n_2=100\Rightarrow n_2=100-n_1$
$\overline{x_i}=$  Average mark of boys $=52,\overline{x_2}=$  Average mark of girls $=42$ 
$\overline{x}=$  Average mark of boys and girls  $=50.$
Combine mean 

 $$\begin{gathered} \overline{x}=\frac{n_1{\overline{x}}_1+n_2{\overline{x}}_2}{n_1+n_2} \\ \Rightarrow 50=\frac{n_1.52+(100-n_1)42}{100} \\ \Rightarrow n_1=80. \end{gathered}$$