Verified Answer :

2nd degree and in form
ax^2+bx+c
it is a prabola
the minimum cost is the vertex (every parabola has only one, min or max)
minimum cost is the vertex

vertex=-b/(2a)

a=0.4
b=-208
vertex=-(-208)/(2*0.4)
vertex=208/0.8
vertex=260 (means 260 dresses)
that is x value
subsitute for y value
C(x)=0.4(260)^2-208(260)+37365
C(x)=10325

minimum cost is $10,325