In this question , n represent the large packages . And total number of packages =12.
So , total number of small packages=12-n
Cost of large packages=$7 and of small packages=$4.50 .
So total cost is the sum of cost of large packages and cost of small packages .
That is
[tex] C(n) = 7n +4.50(12-n)
\\
C(n) = 7n+54-4.50n
\\
C(n)= 2.50n +54 [/tex]
And in the second part, n=8 .
Substituting 8 for n, we will get
[tex] C(8)= 2.50(8) +54 = $74 [/tex]