well, we don't know the cost of the carpet, however hmmm we know the cost of the carpet is 4 bucks per ft², which means we need to know how many ft² the Randall family is going to use in the first place hmmmm, Check the picture below.
[tex]\textit{area of a trapezoid}\\\\ A=\cfrac{h(a+b)}{2}~~ \begin{cases} h=height\\ a,b=\stackrel{parallel~sides}{bases}\\[-0.5em] \hrulefill\\ a=12\\ b=15\\ h=10 \end{cases}\implies \begin{array}{llll} A=\cfrac{10(12+15)}{2} \\\\\\ A=\cfrac{10(27)}{2}\implies A=135~ft^2 \end{array} \\\\[-0.35em] ~\dotfill\\\\ \stackrel{per~ft^2}{(4)}~~\stackrel{area~in~ft^2}{(135)}\implies \stackrel{\$}{540}\impliedby \textit{total cost}[/tex]
