Beginning Algebra With Applications, Chapter 4, 4.1, Section 4.1, Problem 44
A carpenter is building a wood door frame. The height of the frame is 1 ft less than three times the width. What is the width of the largest door frame that can be constructed from a board 19 ft long?
If we let $h$ and $w$ be the height and width of the door frame respectively, then we get
$h = 3w-1 \qquad$ Equation 1
And
$w+2h = 19 \qquad$ Equation 2
By substituting equation 1 to equation 2, we have
$
\begin{equation}
\begin{aligned}
w+2 (3w-1) =& 19
\\
w+2 (3w) - 2(1) =& 19
\\
w+6w -2 =& 19
\\
7w =& 21
\\
w =& 3
\end{aligned}
\end{equation}
$
Thus, the width of the door's frame is 3 ft.
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