Question

The surface area of a swimming pool is 210 square feet. What is the length d of the deep end (in feet)?

Answer 1

$\bf \textit{area of a rectangle}\\\\ A=Lw~~ \begin{cases} L=length\\ w=width\\[-0.5em] \hrulefill\\ A=210\\ w=10\\ L=d+9 \end{cases}\implies 210=(d+9)(10)\implies \cfrac{210}{10}=d+9 \\\\\\ 21=d+9\implies 12=d$