Answer
$490 \text{ mph}$
Work Step by Step
Let $x$ be the speed of the car. Then $x+430$ is the speed of the jet.
Using $D=rt,$ then for the car,
\begin{array}{l}\require{cancel}
210=xt
\\
\dfrac{210}{x}=t
.\end{array}
Using $D=rt,$ then for the jet,
\begin{array}{l}\require{cancel}
1715=(x+430)t
\\
\dfrac{1715}{x+430}=t
.\end{array}
Equating the two expressions of $t$ and using the properties of equality, then
\begin{array}{l}\require{cancel}
\dfrac{210}{x}=\dfrac{1715}{x+430}
\\
210(x+430)=x(1715)
\\
210x+90300=1715x
\\
210x-1715x=-90300
\\
-1505x=-90300
\\
x=\dfrac{-90300}{-1505}
\\
x=60
.\end{array}
Hence, the speed of the jet, $x+430$, $x,$ is $
490 \text{ mph}
.$
