You are given that the Macaulay duration of a twenty-year level unit annuity immediate, whose present value is denoted by a20 , is 8.90, while the Macaulay duration of a standard increasing twenty-year annuity immediate, whose present value is denoted by (Ia)20 , is 12.47. Given that, find the Macaulay duration of a standard decreasing twenty-year annuity immediate, whose present value is denoted by (Da)20.