Question

A net downward force of 20 N causes a book to fall towards the ground at m/s2. What is the book’s mass? Consider air resistance

Answer 1

If we have to figure air resistance into it, then we don't have enough information to find an answer.  

If the air around it is going to have an effect on how it falls, then it'll depend on the thickness of the book, the shape of the book, whether it's a hard-cover or soft-cover, how far the covers stick out past the pages, how smooth or rough the covers are, how bumpy the binding it. and what position you hold it in before you let it go.

(THAT's why we always ignore air resistance, especially when the question is actually about gravity anyway.)