Question 972371
They are traveling along two legs of a right triangle with the hypotenuse being the closest distance between them.

500^2=250,000

(300x)^2 +(200x^2)=250,000

90,000 x^2 + 40,000 x^2=250,000

130,000X^2=250,000
x^2=1.923
x=1.38 hours  rounding eight places to 84 minutes (being nearest minute they won't be able).

Check:
Plane A in 84 minutes will travel 420 miles
Plane B in 84 minutes will travel 280 miles

420^2=176,400
280^2=78,400
sum is 254800

check 83 minutes
415^2 + 276.677^2=248,975

84 minutes.