Catherine Dohanyos plans to retire in 20 years. She will make 20 years of monthly contributions to her retirement account. One month after her last contribution, she will begin the first of 10 years of withdrawals. She wants to withdraw $2800 per month. How large must her monthly contributions be in order to accomplish her goal if the account earns interest of 7.7% compounded monthly for the duration of her contributions and the 120 months of withdrawals?

The amount of her monthly contributions must be?