Math Help for Section 3.5, Page 141

Example
6: Tip
The simplest way to think of this problem is to consider unit analysis.
It asks for an answer in gallons per minute, so the natural thing to do
is divide the number of gallons by the number of minutes. No other
simple operation involving the two known quantities would result in an
answer with the correct units. Here is a more detailed version of the
verbal model.

    $\eqalign{{\text{Rate of drainage}}
&= \displaystyle{{\text{Volume of  pool (gallons) }}
\over
{\text{Time (minutes)}}}\cr &=
\displaystyle{{\text{Volume of pool }} \over {\text{Time}}}{\text{ (gallons per minute)}}}$

Example
6: Check

    $\eqalign{ 15,600
=& r(390)&{\small\color{red}\quad\quad\text{Write
original
equation.}} \cr 15,600\overset{?}{=}&
\color{red}40\color{black}
\bullet
390&{\small\color{red}\quad\quad\text{Substitute 40
for }r.}  \cr 15,600 =&
15,600&{\small\color{red}\quad\quad\text{Solution
checks. }\checkmark}\cr} $