Uncle Zhang and Uncle Li earned a total of 800×(7+6)= 10400 yuan in June.
(2) Science book 2 100×5/7= 1500, story book 2100-1500 = 600;
Later, there were 600÷2/9 = 2700 science and technology books.
So I later bought 2700- 1500= 1200 science and technology books.