A 2n-year annuity has deposits of X made at the end of each of the first n years and deposits of 2X made at the end of each of the next n years. The accumulated value of this annuity at the end of 2n years is 6000. Suppose that the annual effective rate of interest is i, where i > 0. You are given X i = 1500. Find the present value of this annuity at time t = 0.