First let's assume that d0 miles is initial distance between two planes. 1. Both planes are traveling toward to each other with speed (500 + (500-45)) = 955 mph, and they will pass each other in (d0/955) hours. 2. Both planes are traveling in opposite directions with 955 mph and they will never "meet" each other. 3. Plane a "tries to catch" plane b and it will happen in d0/(500-455) = d0/45 hours. 4. Plane b (slower moving one) "tries to catch" plane a .... and it will never happen.