SAT Question ad1d6adb: Percentages answer and explanation with Desmos solution

Choice D is correct. It's given that the number of coins in the collection increased from $9$ to $90$. It follows that the number of coins in the collection increased by $90-9$, or $81$. Let $x\%$ represent the percentage that $81$ is of $9$. The value of $x$ can be found using the proportion $\frac{81}{9}=\frac{x}{100}$, or $9=\frac{x}{100}$. Multiplying both sides of this equation by $100$ yields $900=x$. Thus, when the number of coins in the collection increased from $9$ to $90$, the percent increase was $900\%$.