At an annual effective interest rate of i, i > 0%, the present value of a perpetuity paying 20 at the end of each 3-year period, with the first payment at the end of year 3, is 39.

At the same annual effective rate of i, the present value of a perpetuity paying 1 at the end of each 4-month period, with first payment at the end of 4 months, is X. Calculate X.