It would take an hour for plane a to meet plane b which is flying towards it. if the distance between them will be 200 miles after 45 minutes, how far apart are they now?

Let a = speed of the plane A, and b = speed of plane B.
(Assume that the net effect of the wind is negligible.)
From the first sentence, the distance between A and B after each travels for 1 hour is a + b miles. From the second sentence, after 45 minutes, or 0.75 hour, the distance between A and B is 200 miles. Plane a has traveled 0.75a, while plane B has traveled 0.75b.
Hence a + b = 0.75a + 0.75b +200
==> 0.25(a + b) = 200.
==> a + b = 800.
Therefore planes A and B are 800 miles apart now.