The long-run price is the minimum average total cost and it is found by equating the first differentiation of ATC equal to zero.
The price is $87.75
b)
Q=6000-P
P=87.75
Q=6000-87.75
Q=5912.25
the total industry output is 5912.25 units
c)
Number of firms = total industry output /one firm output =5912.25/1.5
=3941.5
The number of firms is 3941.5
