Let's call p pots and d dishes.
We know that the sets of pots cost $30 and the set of dished cost $20, we express this as the following
[tex]30p+20d=700[/tex]Since the target sales are $700.
The total number of pots and dishes people bought were 30 sets, we express this as the following.
[tex]p+d=30[/tex]We can form a system of equations we these equations.
[tex]\mleft\{\begin{aligned}30p+20d=700 \\ p+d=30\end{aligned}\mright.[/tex]To solve this, we multiply the second equation by -20, then we sum equations to eliminate d and solve for p.
[tex]\begin{gathered} \mleft\{\begin{aligned}30p+20d=700 \\ -20p-20d=-600\end{aligned}\rightarrow10p=100\mright. \\ p=\frac{100}{10}=10 \end{gathered}[/tex]Now, we find the sets of dishes sold.
[tex]\begin{gathered} p+d=30 \\ 10+d=30 \\ d=30-10 \\ d=20 \end{gathered}[/tex]