Two buses are moving towards each other, one at a speed of 40 mph and the other at a speed of 50 mph. How much closer to each other will they be in one hour? In two hours? In three hours?
They will be closer by ten hours but farther by ten as more hours pass by. So you forgot a starting distance of where it starts. If this is needed to be answered specifically, you'll need more information to support answers. Now what I told you is the straying answer.