1)
C(n)=Total cost
C(n)=4.5n+400
Answer: C(n)=4.5n+400
2)
I(n)=incomes
I(n)=12n
Answer: I(n)=12n
3)I(n)>C(n)
Therefore:
12n>4.5n+400
12n-4.5n>400
7.5n>400
n>400/7.5
n>53.333...;
Therefore: n>54
Answer: The inequality would be: 12n>4.5n+400; The school orchestra would have to sell more than 54 CDs.