The primary area of the brain she is using is called the frontal cortex. It works combined with the hippocampus.
The frontal cortex or frontal lobe refers to a region of the cerebral cortex in the brain, which can be divided into the medial and lateral frontal cortex.
The frontal cortex is involved in major executive functions including, among others, cognitive skills, working memory, etc.
The frontal cortex works combined with the hippocampus to solve arithmetic mathematical problems.
Learn more about the frontal cortex here:
https://brainly.com/question/7304463