Basic linear programming question
Suggestions:

Varaibles:

introducing a dummy varaible $t$.

Objective function:

add term $-20t $ to the function

Constraints:

Replacing the last constraint w.r.t. Machine C with 
\begin{align}
t \leq 16 \\
2x_1 + 1x_2 \leq t \\
t\geq 0.
\end{align}
Done !