Algorithm expert

We have a source code project to go over files (for example .txt files) and count the characters of each type "a", "b", "c". The text should be split into sentences. We want to set a target (how many "a"s, how many "b"s, etc.) and the software should add selected sentences which sum up to the target or +- 5% of it. It the change needed is mostly some sort of optimization of selecting the exact amount of sentences which will have in total the requested totals of each character.