With each 12 minutes the trains travel 20 miles thus the distance between trains is reduced by 40. The bee as per in the question travels to and back from a train however in calculation we find that with each 12 min of travel of bee it travels 200 miles while the distance between trains has reduced by 40 thus in the next travel the bee will have to travel 160 miles and reducing. This too is a bit incorrect i.e. while the bee travels to one train at the same time the train travels towards the bee thus it takes the bee about 11.3 something like this minutes in which it travels about 17. something miles less than the whole 200 miles. Taken into account this factor makes the answer vary a lot i.e. the answer 1000 is incorrect rather it should be something in the 600-700 mile range.
No, Every 12 minutes time bee travels 200 miles because the speed of bee is 1000 mph and travel of bee does not effect by the distance between train.