|Math puzzles might help a person's thinking skills, but a person who is doing them instead of work is not thinking clearly at all. She has a job to do, and if she is not doing it, you need to take corrective action.|
She is probably trying to take advantage of your inexperience. If you do not take action to stop the math games, she is going to continue them and probably play all sorts of other games in her dealings with you. Her explanation is merely a rationalization, and it is not acceptable.
You should meet with her in private and let her know that she cannot continue any non-work activity when she is supposed to be working. Be sure to document the specific behavior and dates, let her know the consequences if the problem continues, have her sign the documentation, and then place it in her file.
Looked at in mathematical terms, she needs to understand that her current actions do not compute, and unless she takes immediate corrective action, she is going to do a real number on her career.