Answer:
a)

b)

Explanation:
From the question we are told that:
Rate r=90 units per hour
Setup cost =20
Operating Cost =26
Units=40000
Generally the equation for Total cost is mathematically given by


Differentiating

Equating equ 1 to zero


Therefore
Substituting n
For Equ 1

F(n)>0
For Equ 2

F(n)'<0