.
This part of your question " C(90,00) " is written in absurdist format,
which is not consistent with the rest of the question and makes ANY SOLUTION nonsensical.
Please revise your question, fix it, write appropriately and then re-submit to the forum.
Do not accept any solution, until you present the problem in the proper form.
/////////////
O-o-o (!)
After my post, somebody shamefacedly just fixed the numbers in your post.
Now the solution is in one line
C(90,000) = 0.3*90000 + 27000 = 54,000 dollars. ANSWER
Solved.
Do not accept any other answer.
............
Hey, the solution is so simple: substitute and calculate.
Can you reveal me your secret --- for what reason did you post so simple task to the forum ?