Since the distances were the same you can find the average speed directly by adding the speeds then dividing by 2. No need to weight anything.
If we’re considering speed regardless of direction, the average speed is:
(24 + 16) / 2 = 40 / 2 = 20mph
However if direction matters, then one number has to be negative since the directions are opposite:
(24 - 16) / 2 = 8 / 2 = 4mph (in the positive direction)
Depending on the context, either of these could be your answer.
