pronoun agreement: compound antecedents with and

Pronouns have to agree in person, number and gender with the words they refer to (called their antecedents). But when a pronoun has two or more antecedents joined by and, some special rules apply.

Two or more antecedents joined by and almost always express a plural idea. Therefore, the pronoun referring to them is normally plural:

• The mother and daughter had a good time when they drove across country.

However, if the antecedents are preceded by each or every, they take on a singular meaning. In that case, the pronoun referring to them must be singular:

• Each man and boy lined up to get his food rations. 
• Every wall and pillar had its surface covered in graffiti.

Also, some antecedents joined by and may actually express a single idea and take a singular pronoun:

• Drinking and driving often brings its own punishment. [Drinking and driving (i.e. driving while drunk) is a single action.]