Grammatical Number – Is ‘Audience’ Singular or Plural?

Short answer: in contemporary English, both USA and the long form United States of America are treated as singular nouns.

Long answer: Language Log has documented this in great detail. In the 18th and much of the 19th centuries United States was treated as plural, but in the latter half of the 19th century the singular usage became more common. Today, the singular usage is the only accepted usage, except for the case of a few fixed phrases. In fact, "in 1902 article in the Washington Post reported that Foster's work (which evidently was reprinted as a pamphlet) had persuaded the House of Representative's Committee on Revision of the Laws to rule that the United States should be treated as singular, not plural."

You should use singular in this case.

There is only one units digit in each multiple, and it is 0.

Much the same can be accomplished in any complex network:

• Everyone in our branch offices has an accountant as a supervisor.
• All the people in our branch offices have an accountant as a supervisor.

The difference between the singular has and plural have is because every is singular, while all is plural (I don't make up these rules). But there is only one supervisor in each case, and it's an accountant who's the supervisor.