#### Answer

$x=\left\{ 50,70 \right\}$

#### Work Step by Step

Equating each factor to zero (Zero Product Principle), then the solutions to the equation, $
(20-0.4x)(7-0.1x)=0
,$ is
\begin{array}{l}\require{cancel}
20-0.4x=0
\\\\
-0.4x=-20
\\\\
x=\dfrac{-20}{-0.4}
\\\\
x=50
,\\\\\text{OR}\\\\
7-0.1x=0
\\\\
-0.1x=-7
\\\\
x=\dfrac{-7}{-0.1}
\\\\
x=70
.\end{array}
Hence, $
x=\left\{ 50,70 \right\}
.$