Average is nothing but sum of elements divided by no.of elements.
Average = Sum of all quantities / No.of quantities.
In other terms,
Average = Sum.Of.Observations / No.Of.Observations
Sum.Of.Obserations = Average x No.Of.Observations
No.Of.Observations = Average / Sum.Of.Observations

Let an object travels same distance twice with different speeds as x km/hr and y km/hr.
Average Speed = 2xy/(x+y) =Total Distance/Total Time
Let A is the average of M quantities and B is the average of N quantities.
Weighted Average = Sum.Of.Weighted.Terms / No.Of.Weighted.Terms = [(A x M) + (B x N)]/[M + N]
Note: Average can be calculated on similar quantities not dissimilar quantities.
Average time can be calculated.
Average distances can be calculated.
But average of time and distance can't be calculated.