General Usage of pronouns:-

1. When two singular nouns joined by 'ans' and are preceded by 'each' or every' the pronoun must be singular in number.

every student and every teacher took his seat.

2. When two singular nouns are joined by 'and' denoting the same person/thing, the person/ thing , the pronoun used for them must be singular in number.

the definite article 'the is placed before the first noun.

The accounts office and treasurer should be careful in his work .

3.When nouns of different genders are combined by a conjuction, the pronoun must agree with the gender of the noun which is next to the conjuction.

Every boy and girl went to her house