In The Grammar Book by Celce-Murcia and Larsen-freeman (1983), one of the general rules of subject-verb agreement is described as follows:
"Fractions and percentages take the singular when they modify a mass noun and the plural when they modify a plural noun; either the singular or the plural may be used when they modify a collective noun." (pp.38-9)
Ex:
One-half of the toxic waste has escaped.
Fifty percent of the toxic waste has escaped.
Two-third of the students are satisfied with the class.
Sixty-six percent of the students are satisfied with the class.
One tenth of the population of Egypt is/are Christian.
Ten percent of the population of Egypt is/are Christian.
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