$3 \mathrm{~g}$ of every $40 \mathrm{~g}$ helping of cereal is fat. (c) Write $3 \mathrm{~g}$ as a percentage of $40 \mathrm{~g}$.

Step 1 ：Divide the amount of fat (3g) by the total amount of cereal (40g): \(\frac{3}{40}\)

Step 2 ：Multiply the result by 100 to find the percentage: \(\frac{3}{40} \times 100 = 7.5\%\)

Step 3 ：\(\boxed{3g \text{ of fat is } 7.5\% \text{ of the } 40g \text{ helping of cereal}}\)