When Sarah turned 18, she took $500 from her savings and opened a
ROTH IRA. Sarah chose to invest her money in a low fee index fund.
Based on historical data, Sarah predicts she can expect an average 7%
return on her money each year. If Sarah's prediction is correct, how
much will her $500 investment be worth when she is 68 years old?
(Assume she makes no other contributions to the account after the first
time)