The rule is that if you have a singular subject, you must have a singular verb. (And if you have a plural subject, the verb must be plural.) We call it subject-verb agreement. Take a look at this sentence:
By July 15, an average of 2,500 tons of supplies was being flown into the city every day.
It’s from a passage in This Day in History for June 26. Is the sentence correct or not?
It’s an easy sentence to get wrong, but they got it right! The subject is average, not tons, and not supplies. The latter two words are objects of prepositions, so neither can be the subject of the sentence. So average has to be the subject.
Be careful out there. Those prepositional objects’ll get you if you’re not alert.