Certain nouns and pronouns called collectives may take either plural or singular verbs as predicates, depending on the meaning being conveyed. If the noun or pronoun is viewed collectively as a unit or expresses an amount or measure, its verb is singular; if viewed as separate items, the verb should be plural.
Examples (correct)
About 3000 tons of oil was produced. (A quantity weighting, in all, about 3000 tons).
At this place, 10 meters of sandstone is exposed. (A single exposure).
Twenty-two grams of sample was dissolved in 20 ml of acid. (A single measurement)
Four-fifths of the country is satisfied with its health insurance. (One part of a whole)
Before 1940, less than 10% of production was from opencast mines (Again one part of a whole)
Twenty thousand US dollars is less than the real cost is (A single sum of money)
The United States protects its natural resources. (One country)
Three samples were placed on the table. (Three individual samples).