Dim str
str =""
Dim num
Dim ans
Dim bln_Confirm
Function main()
str = InputBox("数値を入力",,str)
If instr(str,"_") Then
ans = Eval("(" & Replace(str,"_","+") & ")/" & UBound(split(str,"_"))+1)
else
ans = Eval(str)
End If
bln_Confirm = MsgBox("入力数値:" & str & vblf & vblf & "入力の平均:" & ans,3,"続ける?")
str = str & "_"
End Function
Do
main
Loop While bln_Confirm = vbYes

VBSならこうかな？