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 

Let $n_1$ and $n_2$ be the number of boys and girls respectively in the class.

We have $\overline{X_1}=52,\overline{X_2}=42$ and $\overline{X}=50$

$\begin{array}{l}\therefore \overline{X}=\frac{n_1\overline{X_1}+n_2\overline{X_2}}{n_1+n_2}\\ \Rightarrow 50=\frac{52n_1+42n_2}{n_1+n_2}\Rightarrow 8n_2=2n_1\Rightarrow n_1=4n_2\end{array}$

$\therefore$Percentage of boys $=\frac{n_1}{n_1+n_2}\times 100=\frac{4n_2}{5n_2}\times 100=80$