Step 1: Check feasibility:
$(4, 3)$: $3(4) + 3 = 15 \leq 15$ and $12 + 12 = 24 \leq 24$. Both hold, so it is a feasible corner.
Step 2: Compare values:
Corner values are $0, 20, 30, 31$. The largest is $31$.
The options $42$ and $47$ are not reached anywhere in the region.
Final Answer:
The maximum value of $Z$ is $31$, option (A).
\[ \boxed{31} \]