Adding index based on another Column value (Add Sequential Index to Grouped Data)

in such situation, adding an index column and filtering the rows with the index column less than the index value of on that rows and then filtering the text over extracted rows could be usefull.

consider this table

after adding index column using the below formula can be filtered all the previous rows with the value equal to "A" on the first column.

Table.SelectRows(#"Added Index", (X)=> x[Index]<=_[Index] and x[A]=_[A])

instead of showing them, we can use Table.RowCoun and solve the problem as you whanted. so the whole solution provided as below

let
    Source = Table.FromRecords({
[A = "A", B = 2],
[A = "B", B = 10],
[A = "A", B = 10],
[A = "A", B = 10],
[A = "A", B = 10],
[A = "A", B = 10],
[A = "A", B = 10],
[A = "A", B = 10],
[A = "5", B = 10],
[A = "5", B = 10],
[A = "05", B = 10],
[A = "05", B = 10],
[A = "05", B = 10],
[A = "B", B = 10],
[A = "B", B = 10],
[A = "5", B = 10]
}),
    #"Added Index" = Table.AddIndexColumn(Source, "Index", 0, 1, Int64.Type),
    #"Added Custom" = Table.AddColumn(#"Added Index", "Custom", each Table.RowCount(Table.SelectRows(#"Added Index", (x)=> x[Index]<=_[Index] and x[A]=_[A])))
in
    #"Added Custom"