At 4.00 AM the temperature was $12^{\circ}$ below zero. By noon it raised to $30^{\circ}$. What was the average hourly increase in temperature ?

Question

$\begin{array}{1 1} (a)\; 5^{\circ} \\ (b)\; 5.25^{\circ} \\ (c)\; 5.5^{\circ} \\ (d)\; 5.75^{\circ} \end{array}$