Thank you. However, I still don't quite understand what | means,
And that's the problem. Without knowing what it means you're left with guesswork. I remember modern maths when I was at school, things like matrices left my parents cold.
This could be a simple arithmetical problem or it could be something else you're just not familiar with because you, (and I,) weren't taught it. It may have something to do with coding or sequencing or even algorithms. The best thing the OP could do is talk to her niece's teacher.