A woman has a taco stand. She found that her daily costs are approximated by $C(x)=x^{2}-40 x+620$, where $C(x)$ is the cost, in dollars, to sell $x$ units of tacos. Find the number of units of tacos she should sell to minimize her costs. What is the minimum cost?

Step 1 ：The problem is asking for the minimum cost, which means we need to find the minimum point of the function \(C(x)=x^{2}-40 x+620\). This is a quadratic function, and the minimum point of a quadratic function is given by the vertex.

Step 2 ：The x-coordinate of the vertex of a quadratic function \(f(x) = ax^2 + bx + c\) is given by \(-\frac{b}{2a}\). In this case, \(a=1\) and \(b=-40\).

Step 3 ：So, we need to calculate \(-\frac{-40}{2*1}\) to find the number of units of tacos she should sell to minimize her costs.

Step 4 ：After that, we can substitute this value into the function to find the minimum cost.

Step 5 ：Final Answer: The number of units of tacos she should sell to minimize her costs is \(\boxed{20}\). The minimum cost is \(\boxed{220}\) dollars.