A woman has a taco stand. she found that her daily costs are approximated by upper c left parenthesis x right parenthesis equals x squared minus 40 x plus 660c(x)=x2?40x+660 where? c(x) is the cost in dollars to sell x units of tacos. ?a) find the number of tacos she should sell to minimize her costs. ?b) what is the minimum? cost?