For maths problems, is that right to say, 'I can't work it out in my head.'? How about "mind", or "brains"? Thanks you.
The way you have stated it is commonly heard. Iīve never heard either mind or brains used in this context, however mind is sometimes used in other types of situations, e.g. "His actions were so distressing. I canīt work them out in my mind." (I can`t understand his bad behavior.)