#### Answer

$x=300$

#### Work Step by Step

Using the properties of equality, the solution to the given equation, $
0.05x+0.03(1200-x)=42
,$ is
\begin{array}{l}\require{cancel}
100\left[ 0.05x+0.03(1200-x) \right]=(42)100
\\\\
5x+3(1200-x)=4200
\\\\
5x+3600-3x=4200
\\\\
5x-3x=4200-3600
\\\\
2x=600
\\\\
x=\dfrac{600}{2}
\\\\
x=300
.\end{array}