Question 33462
You can use the distance formula for this problem:
{{{d = rt}}} where:
d = distance traveled, r = rate of travel (speed) and t = time of travel.

For the north-bound plane:
1) {{{d1 = r1(t)}}} r1 = 280 mph
For the south-bound plane:
2) {{{d2 = r2(t)}}} r2 = 320 mph

You want the sum of the distances (d1 + d2) to equal 1,000 miles.

From equation 1) you get:
{{{d1 = 280t}}}
From equation 2) you get:
{{{d2 = 320t}}} 

Add the two d's.
{{{280t + 320t = 1000}}} Simplify and solve for t.
{{{600t = 1000}}} Divide both sides of the equation by 600
{{{t = 1000/600}}}
{{{t = 1.67}}}hours.

t = 1 hour and 40 minutes.