Question 819906
You want to find the rate for each.
r= rate for the CAR.


Vehicle__________speed____________time_______________distance
TRUCK____________r-30_____________t__________________315
CAR______________r________________t__________________518


Using rate*time=distance, get the time expressions for each:


Vehicle__________speed____________time_______________distance
TRUCK____________r-30_____________{{{315/(r-30)}}}__________________315
CAR______________r________________{{{518/r}}}__________________518


Remember from description, those times are equal.
{{{highlight_green(315/(r-30)=518/r)}}}
{{{518(r-30)=315r}}}
{{{518r-518*30=315r}}}
{{{518r-315r=518*30}}}
{{{203r=15540}}}
{{{r=15540/203}}}
{{{highlight(r=76.55)}}} or close enough to 77 miles per hour for the car.
The truck goes at 47 mph.