SOLUTION
Given the question in the image, the following are the solution steps to answer the question.
STEP 1: Write the given equations
[tex]\begin{gathered} C=25y+10-----equation\text{ 1} \\ y=30x+15----equation\text{ 2} \end{gathered}[/tex]STEP 2: Substitute 30x+15 for y in equation 1
[tex]\begin{gathered} C=25(30x+15)+10 \\ C=750x+375+10 \\ C=750x+385 \end{gathered}[/tex]Hence, the answer is:
[tex]C=750x+385[/tex]