So.... notice the picture below
we know when y = 140 and x = 240, r = 250
so let's use the pythagorean theorem

now... how much time does the controller have? well, say, plane "y" is going at dy/dt of 140mph, and is 70 miles away, 140 is 2*70, so it can cover those 70 miles in 1/2 hr, and when it does, it'll collide with the other,
so, the controller has that much, less than 1/2 hr
Hence, the distance between (7,8) and (-8,0) is:17
explanation: (a,b)=(7,8) and (c,d)=(-8,0)
it would be a, b, and c because if you look at the question and the answers, 3 of the four answers match the 3 of the 4 coordinates
Let the number = x
Then, it would be: (x*36)/100 = 108
x*36 = 10800
x = 10800/36
x = 300