Option Explicit
Sub Transformation_HHMMSS()
Dim Cell As Range
Dim TmpMM As String
Dim TmpSS As String
Dim Container As Variant
Dim WS As Worksheet
For Each WS In ActiveWorkbook.Worksheets
With WS
For Each Cell In Union(.Range('B4:B99'), .Range('C4:C99'))
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 Cell
With .Range('B100')
.Formula = '=AVERAGE(B4:B88)'
.NumberFormat = 'hh:mm:ss'
End With
With .Range('C100')
.Formula = '=AVERAGE(C4:C88)'
.NumberFormat = 'hh:mm:ss'
End With
End With
Next WS
End Sub