Accumulated Average explained

For example, if I have a set of data like this:

5, 10, 15, 20 -

Then, I would want to get Accumulated Average of this set as

5, 7.5, 10, 12.5

Where
1st data is 5, as 5 is the only number so far.
2nd data is 7.5, as (5+10) / 2
3rd data is 10 as (5+10+15)/3
4td data is 12.5 as (5+10+15+20)/4

Source:
www.devexpress.com/Support/Center/p/Q242748.aspx