$15$ is $60\%$ of what number?

Let $z$ be the unknown whole. "$15$ is $60\%$ of $z$" means $15 = 0.60 \cdot z$. Divide both sides by $0.60$: $z = \dfrac{15}{0.60} = 25$. Check: $60\%$ of $25 = 0.60 \times 25 = 15$ ✓. Alternative (proportion): $\dfrac{15}{z} = \dfrac{60}{100}$, so $60z = 1500$, $z = 25$. - Trap A ($9 = 60\% \text{ of } 15$) inverts the question — it takes $60\%$ of $15$ instead of asking *what* $15$ is $60\%$ of. - Trap D ($90$) divides by $1/6$ instead of $0.6$.