Write the original ratio as a fraction: sugar over flour is $\frac{2}{5}$. Then write another fraction with 3 as the sugar amount and $x$ as the flour amount.

Once you have $\frac{2}{5} = \frac{3}{x}$, use cross-multiplication to solve for $x$, then divide to find the number of cups of flour.

In Desmos, type 3 * (5/2) to represent 3 cups of sugar times the flour-to-sugar ratio $\frac{5}{2}$. The value that Desmos outputs is the number of cups of flour needed to keep the same ratio.

The ratio $2:5$ (sugar:flour) means:

• For every 2 equal-volume parts of sugar,
• There are 5 equal-volume parts of flour.

So flour is the larger part, and it is always $\frac{5}{2}$ times as much as the sugar.

We are told the baker now uses 3 cups of sugar. Let $x$ be the number of cups of flour needed.

Set up the proportion so that sugar is on top and flour is on the bottom in both fractions:

This says: "2 parts sugar for 5 parts flour" is the same as "3 cups sugar for $x$ cups flour."

Solve the equation

by cross-multiplying:

So the baker should use 7.5 cups of flour, which corresponds to answer choice C.