College Algebra, Chapter 1, 1.2, Section 1.2, Problem 30

Suppose that a father is four times as old as his daughter. In 6 years he will be three times as old as she is. How old is the daughter now?

If we let $x$ be the age of the daughter, the age of the father is $4x$

$
\begin{array}{|c|c|c|}
\hline\\
& \text{Present} & \text{Future} \\
\hline\\
\text{Father} & 4x & 4x + 6 \\
\hline\\
\text{Daughter} & x & x + 6\\
\hline
\end{array}
$

So,


$
\begin{equation}
\begin{aligned}

4x + 6 =& 3(x + 6)
&& \text{Model}
\\
\\
4x + 6 =& 3x + 18
\text{Simplify}
\\
\\
x =& 12
&&

\end{aligned}
\end{equation}
$



Therefore, the present age of the daughter is 12.