A mother opened an investment account for her son on the day he was born, investing $$800.$ Each year on his birthday, she deposits $$800,$ making the last deposit on his $20$ th birthday. If the account paid a return rate of $5\mathrm{.}7\%$ compounded annually, how much is in the account at the end of the day on the son's $20$ th birthday?

The mother's deposits form $an,$ because the deposits are made $at$ the $,of$ each period. Therefore, the formula

should be used