In a triangle, angle $x$ is three times the size of angle $y$ and $1/2$ the size of $z$. How big is angle $y$ ?

Question

$\begin{array}{1 1} (a)\; 18^{\circ} \\(b)\; 20^{\circ} \\ (c)\; 54^{\circ} \\ (d)\; 60^{\circ} \end{array}$