recall you d = rt, distance = rate * time
when the BMW has been running for 2hrs, the Jeep has been running for 2.5hrs, because it started off the exit 30minutes(0.5hr) before the BMW.
if the rate of the BMW is say "r", then the rate of the Jeep is "r - 7", slower by 7mph.
now, we know after the Jeep was running for 2.5hrs and the BMW was running for 2hrs, they were both apart by 306.5miles, so, say if the Jeep travelled a distance of "d", then the BMW travelled the slack from the 306.5, or "306.5 - d".
[tex]\bf \begin{array}{lccclll}
&distance&rate&
\begin{array}{cllll}
time\\
(hours)\\
\end{array}\\
&-----&-----&-----\\
Jeep&d&r-7&2.5\\
BMW&306.5-d&r&2
\end{array}
\\\\\\
\begin{cases}
\boxed{d}=(r-7)2.5\\
306.5-d=2r\\
----------\\
306.5-\boxed{(r-7)2.5}=2r
\end{cases}[/tex]
solve for "r" to get the rate of the BMW.
what about the Jeep's? well, the Jeep is just r - 7.
