An apartment complex wishes to establish a fund at the end of 2004 that, by the end of the year 2021, will grow to an amount large enough to place new roofs on its 39 apartment units. Each new roof is estimated to cost $2,500 in 2019, at which time 13 apartments will be reroofed. In 2020, another 13 apartments will be reroofed, but the unit cost will be $2,625. The last 13 apartment will be reroofed in 2021 at a unit cost of $2,750. The annual effective interest rate that can be earned on this fund is 4%. How much money each year must be put aside (saved), starting at the end of 2005, to pay for all 39 new roofs? State any assumptions you make.