I have been solving these sort of problems, and they follow this basic paradigm: first, they will tell me I traveled a distance D at a velocity of A, then they traveled the same distance D at a velocity of B. Why can't I simply just simply perform the following computation: (A+B)/2. I have been searching the internet for some unequivocal explanation to this sort of problem, but have found no such explanations that suffice in aiding my comprehension in solving these kinds of problem. So, it would be much appreciated if someone would could give me a good explanation of this.
Thank you, in advance.
Edit: I forgot to mention that the velocity A does not equal the velocity B