(A)
Find the derivative of C(q):
[tex]\begin{gathered} C^{\prime}(q)=0+20(1) \\ C^{\prime}(q)=20 \end{gathered}[/tex](B)
The revenue function is:
[tex]\begin{gathered} R(q)=q\cdot p \\ so: \\ R(q)=q(205-\frac{q}{70}) \\ R(q)=205q-\frac{q^2}{70} \end{gathered}[/tex](C)
The derivative of R(q) is:
[tex]\begin{gathered} R^{\prime}(q)=205(1)-\frac{1}{70}(2q) \\ so: \\ R^{\prime}(q)=205+\frac{q}{35} \end{gathered}[/tex]