A recipe uses 3/4 cup of flour to make 12 cookies. How many cups of flour are needed to make 30 cookies, at the same rate?

The rate is constant, so set up a proportion: $\frac{3/4}{12}=\frac{x}{30}$. Then $x=\frac{3}{4}\cdot\frac{30}{12}=\frac{3}{4}\cdot2.5=1.875$ cups, choice H. As a unit rate, each cookie needs $\frac{3}{4}\div12=0.0625$ cup, and $30(0.0625)=1.875$. Choice F ($1.5$) doubles $\frac{3}{4}$, which is the amount for $24$ cookies. Choice G ($1.75$) is an estimate that does not follow from the rate. Choice J ($2.5$) uses the cookie ratio $\frac{30}{12}$ as if it were the amount of flour. Check that $30$ cookies is $2.5$ times as many as $12$, so the flour is $2.5\cdot\frac{3}{4}$.