At 10:00 a. m. two planes leave an airport. If the northbound plane flies at 280 mph and the southbound at 320 mph, at what time will they be 1,000 miles apart?

After 1.7 hours, they will be 1,000 miles apart.

Step-by-step explanation:

Let t represent the time it will take both planes to be 1000 miles apart.

If both planes leave the airport at 10.00am, it means that they would have covered a total distance of 1000 miles after t hours.

Distance = speed × time

If the northbound plane flies at 280 mph, it means that the distance covered in t hours is 280t.

If the southbound plane flies at 320 mph, it means that the distance covered in t hours is 320t.

Since the total distance covered is 1000 miles, then

280t + 320t = 1000

600t = 1000

t = 1000/600

t = 1.7 hours