Answer
$x=\dfrac{84}{m}$ where x is the cost of gas per mile.
If 200 is substituted to $m$, the cost per mile is $\$0.42$.
Work Step by Step
The cost per mile is:
$\dfrac{84}{m}$
Per indicates that we are dividing.
If he drove a total of 200 miles on one full tank, then he paid:
$\dfrac{84}{200}=0.42$ dollars per mile
The answer makes sense since $200(0.42) = 84$.
