Option Explicit
Sub Transformation_HHMMSS()
Dim Cell As Range
Dim TmpMM As String
Dim TmpSS As String
Dim Container As Variant
For Each Cell In Union(Range('B4:B88'), Range('C4:C88'))
If InStr(1, Cell, 'mn', 1) <> 0 Then
On Error Resume Next
Container = Split(Cell, 'mn')
TmpMM = Val(Container(0))
TmpSS = Val(Container(1))
Cell = '00:' & Format(TmpMM, '00') & ':' & Format(TmpSS, '00')
End If
Next
Range('B100') = Application.WorksheetFunction.Average(Range('B4:B88'))
Range('C100') = Application.WorksheetFunction.Average(Range('C4:C88'))
End Sub