For this case we have:
Cost of manufactures of T-shirts 
where x represents the number of T-shirts
Part A:
Substituting
in the total cost equation you have to:

Thus, the cost of 8 shirts will be 
Part B:
If
then

Thus, the cost of 12 shirts will be 
Answer:

Answer:
$659.25
Step-by-step explanation:
Since its 25 percent off we need to see what it would look as a decimal
1-.25=.75
879times.75= 659.25
Nikkis charges are 20,40,50,80. Dave’s charges are 25 and 37.5.
Example:
38 rounds to 40
6 rounds to 10
10+40 = 50