Answer:
90 engines must be made to minimize the unit cost.
Step-by-step explanation:
Vertex of a quadratic function:
Suppose we have a quadratic function in the following format:

It's vertex is the point 
In which


Where

If a>0, the minimum value of the function will happen for 
C(x)=x²-180x+20,482
This means that 
How many engines must be made to minimize the unit cost?
x value of the vertex. So

90 engines must be made to minimize the unit cost.
Attached is the solution.
The following log properties are used:

5. Yes 1,2, or 3 hours
6. No, because the distance will never change and there aren't multiple values
7. Yes, yes or no (a person could be colorblind)
8. Yes, yes or no (a person could be colorblind)
9. Yes: Rap, country, pop, classical, etc