we are given with a function which represents the costs entailed for her taco stand. The expression given is c(x)=x2-40x+66. to get the minimum cost, we derive the equation and equate to zero. Hence,
c'(x)=0=2x-40
40 = 2x
x=20 tacos
c(x)=20^2-40*20+660
c(x)=$260