Is the singular "they" acceptable in formal writing? [duplicate]
According to Merriam-Webster's Dictionary of English Usage, "they" as a common-gender pronoun is "perfectly well established, even in formal contexts."
If it's growing, it's because of the need, following social developments in the twentieth century, for a gender-neutral pronoun, but it's been used by respected writers for centuries. If you really can't bring yourself to use it, or if you fear it will annoy your readers, then putting the antecedent itself in the plural will often be the answer.