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NUMBER, GRAMMATICAL

See Collectives as Singular or Plural, FRACTIONS, none, and Plurals.

Generally speaking, a number or measurement is singular if considered as a mass:
            ten gallons is enough
            300 miles was a lot to drive in a day

and plural if considered as separate units:
            ten dishfuls were slowly doled out

The object of a preposition following a fraction or percentage usually governs the verb:
            Half the wine is enjoyed ...
            Half of these people are engaged ...
            Two-thirds of humanity lives ...
            35 percent of the bread goes ...
            35 percent of the grapes go ...