And your figure of 900B is a little low. According to wiki, tax
revenue was over 2 trillion in 2009.
http://en.wikipedia.org/wiki/United_States_federal_budget#Federal_budget_data
"During FY 2009, the federal government collected approximately $2.1
trillion in tax revenue. Primary receipt categories included
individual income taxes (43%), Social Security/Social Insurance taxes
(42%), and corporate taxes (7%).[6] Other types included excise,
estate and gift taxes. Tax revenues have averaged approximately 18.3%
of gross domestic product (GDP) over the past 40 years, generally
ranging plus or minus 2% from that level.[7]"
-Bill