Marissa is adding numbers in her head. which part of her brain is she using? 1.cerebrum 2.cerebellum 3.brain stem

Cerebrum is the part of the brain that we use for imagination, thinking, judgement etc. It is also the largest part of the brain 

Cerebellum is in charge of stuff like the spinal cord 

and the brain stem is the part of the brain that helps us move. (if you watch the walking dead in one of the episodes it teaches this xD) 

hope this helped (: