We can use some very simple math to figure out how long it would take to drive 400 miles at 80 miles per hour.
All you need to do is divide the mileage by the speed (400/80), and we can make that even easier by reducing it to 40/8.
When we do that math we find out that 40/8=5. So it would take 5 hours to travel 400 miles at 80 mph.
We can double check the math by asking the question "If I drive for 5 hours at 80mph, how far will I go?" That would be 5 x 80, which equals the 400 from the original question, so we know that we were correct.