I have found the time that they are at the shortest distance to be 1.13 minutes for the first plane and 0.87 minutes for the second plane. I do not know where to continue from there though. I'm also not necessarily sure how to explain that I found the time was 1.13 minutes.
One plane is flying due east straight and level at 30000 feet and at 420 mi/h. A second plane flies due north at 40560 feet and at 480 mi/h. The second plane crosses above the flight path of the first plane 2 minutes after the first plane passes that point. How fast were they approaching each...