Gilly Stevensplans to retire in 15 years. She will make 15 years of semiannual (6 monthly) contributions to her retirement account. One month after her last contribution, she will begin the first of 8 years of withdrawals. She wants to withdraw $900 per month. How large must her semiannual contributions be in order to accomplish her goal if the account earns interest of 8.5% semiannually for the duration of her contributions and 8.1% monthly for the 8 year (96 months) of withdrawals?