She bought 6 books in all because if you divide the whole number of all the books in all which is $32 divide that by 4 so..
32 ÷4=8
so thats just the 4 books then she buys 16 more dollars worth that which you then multiply 4 times 8 which give you 16 so you then add the 4 books to the other 2 books spent on the next day
So basically they are all negatives... positive and negative make negative you add them and put a negative in front.... -25-12-34 add them -37-34 = -71