Solution: Let the number of books purchased for the first time =X, and the purchase price = Y yuan per book, then XY = 100①, the number of books purchased for the second time = 2x, and the purchase price =Y+0.5, then: 2X(Y+0.5)=250② Solution: X=50, Y=2. Pricing = then: 4/5× 2× 50× 2.8+1/5× 2× 50× 2.8×? -250 = 2, earned 2 yuan.