Answer
$50 \text{ } mph$
Work Step by Step
Let $x$ be the speed of the truck. Using $D=rt,$ then, for the truck,
\begin{array}{l}\require{cancel}
450=xt
\\\\
\dfrac{450}{x}=t
.\end{array}
Using $D=rt,$ then, for the plane,
\begin{array}{l}\require{cancel}
450=3x(t-6)
\\\\
\dfrac{450}{3x}=t-6
\\\\
\dfrac{450}{3x}+6=t
.\end{array}
Equating the equations of $t,$ then
\begin{array}{l}\require{cancel}
\dfrac{450}{x}=\dfrac{450}{3x}+6
\\\\
3x\left( \dfrac{450}{x} \right)=\left(\dfrac{450}{3x}+6\right)3x
\\\\
3(450)=1(450)+3x(6)
\\\\
1350=450+18x
\\\\
1350-450=18x
\\\\
900=18x
\\\\
\dfrac{900}{18}=x
\\\\
x=50
.\end{array}
Hence, the speed of the truck, $x,$ is $
50 \text{ } mph
.$
