The price of Grandma Margit's favorite chocolate increased by 30%, while her pension also rose by $15$%. By what percentage will Grandma Margit's chocolate consumption decrease if she can only spend $15$% more on chocolate?
Solution. Let the unit price of chocolate be: $a \mathrm{Ft}$, and the quantity purchased: $x$. Then, Aunt Margit spends $a x \mathrm{Ft}$ on chocolate. The new unit price after the price increase: $1.3 a \mathrm{Ft}$. According to the problem, the amount spent on chocolate after the price increase is $1.15 \cdot a x \mathrm{Ft}$.

Therefore, the quantity purchased after the price increase: $\frac{1.15 a x}{1.3 a} \approx 0.8846 x$, which means Aunt Margit's chocolate consumption decreases by approximately $100-88.46=11.54 \%$.