In the diagram below if $\dfrac{y}{x}=5$ and $\dfrac{z}{x}=4$, then the value of $x$ is

Question

$\begin{array}{1 1} (a)\;8^{\circ} \\ (b)\; 18^{\circ} \\ (c)\; 12^{\circ} \\ (d)\; 15^{\circ}\end{array}$