Hello,

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 :

a=3b

b+50=2(a-50)

You can work with substitutions, the most expeditive way imho