Answer
At an average rate of 50 mph, it will take the car 8 hours to travel 400 miles.
Work Step by Step
RECALL:
For situations involving time (t) , distance (d) and rate (r, sometimes referred to as speed), the formula for time is:
$t=\dfrac{d}{r}$
The given situation has:
$d=400$ miles
$r =50$ mph
Use the formula above to obtain:
$t=\dfrac{400}{50}=8$ hours