A recent college graduate, Robert Williams, is currently working as the manager of an office supply store. He wants to save money for a down payment on a home in a few years, and he's heard that investing in stocks will help him quickly build his investment portfolio. Robert can invest about $2,700 at the end of each of the next three years, and he thinks he'll need to save $18,000 to $23,000 for the down payment. Robert will hold this money in a taxable account, and his marginal tax rate is 15 percent