In a certain class there are 15 girls and 20 boys. If the average height of the girls is $x$ and average height of boy is $y$, what is the average height of all the boys and girls in the class ?

Question

$\begin{array}{1 1} (a)\;\large\frac{x+y}{35} \\ (b)\;\large\frac{20x+15y}{35} \\ (c)\;\large\frac{15x+20y}{35} \\(d)\;\text{none of these}\end{array} $