Câu a: tổng

 Private Function Tinh(n As ULong, m As Integer) As Integer
        Dim j As Integer
        Dim tong As Integer

        If (10 ^ m) > n Then
            MsgBox("Ban da nhap vuot qua so luong chu so", 16, "Error")
        Else
            tong = 0
            n = n Mod (10 ^ m)
            For j = m To 0 Step -1
                tong = tong + (n \ (10 ^ j))
                n = n Mod (10 ^ j)
            Next
        End If
        Tinh = tong
    End Function
    Private Sub CmdIn_Click(sender As Object, e As EventArgs) Handles CmdIn.Click
        Dim n As ULong
        Dim m As Integer
        Do While n <= 0
            n = InputBox("Nhap vao mot so nguyen duong: ", "Nhap", 0)
        Loop
        Do While m <= 0
            m = InputBox("Nhap vao mot so: ", "Nhap", 1)
        Loop
        MsgBox("Ket qua: " & Tinh(n, m), 48, "Ket qua")
    End Sub