I'll try to put mine then
Let a and b be the number of books of A and B.
This means that a=3bA had 3 times as many books as B.
So we're working on a-50 and b+50 because a gave 50 books and b received 50 !A sold B 50 books
-> "number of books now B have is twice number of books A now have"then B had twice as many as A
This means that (b+50)=2(a-50)
Now, you have to solve for a and b with the two equations :
You can work with substitutions, the most expeditive way imho