THEM: can i get a ball, a hat and a book?  YOU: i'm afraid not. i can let you have all the hats if i get the ball and both books.  THEM: cant do that..... ball, hat and book get me 6 / 10  YOU: i need the ball. i can let you have a book and two hats.  THEM: that only gives me 3 / 10..... i'll have to pass  YOU: okay, lets try this. i get both books and a hat. you get the rest.  THEM: ok.