Answer:
The number of watches must he repair to have the lowest cost is 54.
Step-by-step explanation:
The cost of operating Bob's shop is given by
Differentiate the given function with respect to x.
... (1)
Equate C'(x) equal to 0, to find the critical point.
Divide both sides by 4.
Differentiate C'(x) with respect to x.
C''(x)>0, it means the cost of operating is minimum at x=54.
Therefore the number of watches must he repair to have the lowest cost is 54.