If the wholesale price of books purchased for the first time is X yuan, the wholesale price of books purchased for the second time is (x+0.5) yuan.
According to the meaning of the question:100/x+10 =150/(x+0.5)
De: 2x2-9x+ 10=0
Solution: x 1=2.5, x2=2.
After investigation, when x=2.5, the second wholesale price was 2.5+0.5=3 (yuan), which was higher than the price of the book and was discarded.
When x=2, the second wholesale price is 2+0.5=2.5 (yuan), which is lower than the book's pricing and meets the question.
The first time I bought 100÷2=50 copies, and the second time I bought 50+ 10=60 copies.
(60× 4/5× 2.8+60× 1/5× 2.8× 1/2)- 150 = 15 1.2- 150 = 65438+.
Answer: The boss earned 1.2 yuan by buying books for the second time.