Bonjour,
une solution via une macro
Sub aargh()
dl = Cells(Rows.Count, 1).End(xlUp).Row
Range("A1:c" & dl).Sort key1:=Range("B1"), order1:=xlAscending, key2:=Range("A1"), order2:=xlAscending, Header:=xlYes
i = 2
pl = i
While i <= dl
If Cells(i, 2) = Cells(i + 1, 2) Then
If m = 0 Then m = Cells(i + 1, 1) - Cells(i, 1) Else m = (m + Cells(i + 1, 1) - Cells(i, 1)) / 2
i = i + 1
Else
Cells(pl, 4) = m
m = 0
i = i + 1
pl = i
End If
Wend
End Sub