The two cars start from the same point in opposite directions at 40 and 50 mph. The speed at which the two cars are moving away from each other is given by 40 + 50 = 90 mph.
They are 450 miles apart from each other after 450/90 = 5 hours.
As the cars had started at 5 AM, they will be 450 miles apart at 5 + 5 = 10 AM
The required time when the 2 cars will be 450 miles apart is 10 AM.