SAT Question 96a45430: Percentages answer and explanation with Desmos solution

Choice C is correct. The decimal equivalent of $6\%$ is $0.06$. Since increasing the number $n$ by $6\%$ yields the number $318$, this situation can be represented by the equation $n\times (1+0.06)=318$, or $n\times 1.06=318$. Dividing both sides of this equation by $1.06$ yields $n=300$.