Identify part, whole, percent: part = 12, percent = 15%, whole = x. Equation method: $0.15x = 12 \Rightarrow x = 12 \div 0.15 = 80$. Proportion method: $\frac{15}{100} = \frac{12}{x}$, so $15x = 1200$ and $x = 80$. Unit-percent reasoning: if 15% is 12, then 1% is $12 \div 15 = 0.8$, so 100% is $0.8 \times 100 = 80$.

This is find the part: whole = 80, percent = 30%. Mental math: 10% of 80 is 8, so 30% is $3 \times 8 = 24$. Equation: $0.30 \times 80 = 24$. Proportion: $\frac{30}{100} = \frac{\text{part}}{80}$, so part $= \frac{30}{100} \cdot 80 = 24$.

Part = 18, whole = 24, percent = $p%$. The correct proportion is $\frac{\text{part}}{\text{whole}} = \frac{\text{percent}}{100}$, so $\frac{18}{24} = \frac{p}{100}$. Equivalent equation form: $\frac{p}{100} \cdot 24 = 18$. Choice A misses dividing by 100, B reverses part and whole, and D treats $p$ as a fixed fraction unrelated to 18.

This is find the whole: part (discount) = 9, percent = 25%, whole = $x$. Equation: $0.25x = 9 \Rightarrow x = 9 \div 0.25 = 36$. Proportion: $\frac{25}{100} = \frac{9}{x}$, so $25x = 900$ and $x = 36$. Check: 25% of 36 is 9.