You will soon learn that the BGS material is FULL of mistakes!
But then again so are the ACTUAL EXAMS where you have to chose the WRONG ANSWER in order to get the mark correct for that question.
There are some questions in both the BGS learning materials, the BGS Question Bank and the actual exams that are basic maths - and they get the answer wrong.
Thus is the current state of the EASA ATPL exams, and thus the training for them - unfit for purpose.