Since the variable x represents the number of bags of sand, we can set x=0 to represent that no bags of sand are bought.
Replace x=0 into the equation and solve for y to find how many bags of mulch can be bought with $105:
[tex]\begin{gathered} x=0 \\ \\ \Rightarrow3x+7y=3(0)+7y=7y \end{gathered}[/tex][tex]\begin{gathered} 3x+7y=105 \\ \Rightarrow7y=105 \\ \Rightarrow y=\frac{105}{7} \\ \Rightarrow y=15 \\ \end{gathered}[/tex]
Therefore, 15 bags of mulch can be bought with $105. The correct choice is Option C) 15.
