Rewrite each sentence in two ways to clarify the meanings of the pronouns.

Beth and Melissa were both late since she had lost her watch.

The sentence "Beth and Melissa were both late since she had lost her watch" is ambiguous because the pronoun "she" can refer to any of the two girls.

In order to eliminate such ambiguity, we could rewrite the sentence in the following ways:

Beth and Melissa were both late since the former had lost her watch. Beth and Melissa were both late since the latter had lost her watch.

The word "former" is used to indicate the first-mentioned of two people while the word "latter" is used to indicate the last-mentioned of two people.