Ask yourself: by what number do you multiply 3 to get 12? The same number will be used to scale the flour amount.

Take the 5 from the ratio (for flour) and multiply it by the same number you used to turn 3 into 12.

In the Desmos calculator, type 3/5 = 12/x and press Enter. Then either solve for $x$ using the Desmos solver (if available) or note the value of $x$ that makes both sides equal; this $x$ is the needed cups of flour.

Alternatively, in Desmos type 12*(5/3) and press Enter. The output shown is the number of cups of flour that keeps the ratio $3:5$ when using 12 cups of sugar.

The recipe uses sugar and flour in the ratio $3:5$. This means:

• For every 3 parts of sugar, there are 5 parts of flour. So sugar : flour $= 3 : 5$.

The baker will use 12 cups of sugar instead of 3 cups. Find what number multiplies 3 to give 12:

Solve:

So the amounts in the recipe are being multiplied by 4.

Use the same scale factor (4) on the flour amount:

• Original flour in recipe: $5$ parts
• Scaled flour: $5 \times 4$

Compute:

So the baker should use 20 cups of flour to keep the same ratio.