SAT Question 0cb57740: Linear Equations In One Variable answer and explanation with Desmos solution

Choice B is correct. It's given that each side of a $30$ -sided polygon has one of three lengths. It's also given that the number of sides with length $8$

$\text{centimeters}\left(\text{cm}\right)$ is $5$ times the number of sides $n$ with length $3$ cm. Therefore, there are $5\times n$, or $5n$, sides with length $8$ cm. It's also given that there are $6$ sides with length $4$ cm. Therefore, the number of $3$ cm, $4$ cm, and $8$ cm sides are $n$, $6$, and $5n$, respectively. Since there are a total of $30$ sides, the equation $n+6+5n=30$ represents this situation. Combining like terms on the left-hand side of this equation yields $6n+6=30$. Therefore, the equation that must be true for the value of $n$ is $6n+6=30$.