A recipe for 8 servings uses 2 cups of sugar. How much sugar is needed for 20 servings, at the same rate?

The rate is constant, so use a proportion: $\frac{2}{8}=\frac{x}{20}$. Cross-multiply: $8x=40$, so $x=5$ cups, choice B. As a unit rate, each serving needs $\frac{2}{8}=0.25$ cup, and $20(0.25)=5$. Choice A ($4$) scales by $2$ instead of $2.5$, which would be right for $16$ servings. Choice C ($6$) scales by $3$. Choice D ($8$) is the number of servings in the original recipe, not an amount of sugar. Since $20$ is $2.5$ times $8$, the sugar must be $2.5$ times $2$.