20 billion

Chicago citation style
20 billion. 1977-08-29. Retrieved from the Digital Public Library of America, http://ark.digitalcommonwealth.org/ark:/50959/4f16cz32x. (Accessed June 23, 2018.)
APA citation style
(1977-08-29) 20 billion. Retrieved from the Digital Public Library of America, http://ark.digitalcommonwealth.org/ark:/50959/4f16cz32x
MLA citation style
Retrieved from the Digital Public Library of America <http://ark.digitalcommonwealth.org/ark:/50959/4f16cz32x>.
Note: These citations are programmatically generated and may be incomplete.