Jennifer has a taco stand. She has found that her daily costs can be modeled by C(x) = x^2 - 40x + 610, where C(x) is the cost, in dollars, to sell x units of tacos. If she wants to keep her daily costs at $300, how many units of tacos does she need to sell. Give all possible answers, rounding off to nearest whole number. Which answer to part (a) makes the most sense from Jennifer's point of view?