Subject-verb agreement is generally quite straightforward in English. Check any handbook for the general rules. However, with subjects that introduce the idea of quantity, some additional rules of thumb are needed. Here are a few that are useful for academic writing.

With fractions, percentages and indefinite quantifiers (e.g., all, few, many, much, some), the verb agrees with the preceding noun or clause:

With a singular or non-count noun or clause, use a singular verb:

One-third of this article is taken up with statistical analysis.

Much of the book seems relevant to this study.

Half of what he writes is undocumented.

Fifty percent of the job is routine.

All the information is current

With a plural noun, use a plural verb:

One-third of the students have graduate degrees.

Many researchers depend on grants from industry.

Half of his articles are peer-reviewed.

Fifty percent of the computers have CD-ROM drives.

All the studies are current.

With a collective noun, use either a singular or a plural verb, depending on whether you want to emphasize the single group or its individual members:

Half of my family lives/live in Canada.

All of the class is/are here.

Ten percent of the population is/are bilingual.

The words majority and minority are used in a variety of ways:

When majority/minority mean an unspecified number more or less than 50%, use a singular verb:

The majority holds no strong views.

A small minority indicates it supports the proposal.

When majority/minority mean a specific percentage, you may use either a singular or a plural verb:

A 75% majority have/has voted against the measure.

A 10% minority are/is opposed to the measure.

When majority/minority refers to a specified set of persons, use a plural verb:

A majority of Canadians have voted for change.

A minority of the students are willing to pay more.

Expressions of time, money and distance usually take a singular verb:

Ten dollars is a great deal of money to a child.

Ten kilometres is too far to walk.

Six weeks is not long enough.

Adjectives preceded by the and used as plural nouns take a plural verb:

The rich get richer.

The poor face many hardships.

Expressions using the phrase number of depend on the meaning of the phrase:

They take a singular verb when referring to a single quantity:

The number of students registered in the class is 20.

They take plural verbs when they are used as indefinite quantifiers (see rule 1 above):

A number of students were late.

Johnston, M. (n.d.). Expressions of Quantity: Special Cases of Subject-Verb Agreement. Retrieved October 21, 2012, from http://www.writing.utoronto.ca/advice/english-as-a-second-language/expressions-of-quantity