Charlie/Colby

[scudge]

I'm looking for a story where (It might be part of a series) Colby goes home to his family and his niece or something is really good at maths so he calls charlie and it turns out shes a genius and charlie teaches her maths or somthn. I think it was on AO3. That's all I can remember I know its not much but if yall could help that would be awesome.

Thanks in advance