d is a disccount function

and this is an annuity

13. A sum of $10,000 is used to buy a deferred perpetuity-due paying $500 every six months forever. Find an expression for the deferred period expressed as a function of d. 3. A sum of $100 is placed into a fund at the beginning of every other year for eight years, If the fund balance at the end of eight years is $520, find the rate of simple interest eamed by the fund