Calculation method: the total price is 20 minus 4 yuan, and the two prices are 16 yuan. So the price of the pen is 8 yuan, and the book is 8 yuan plus 4 yuan, which is 12 yuan. This is a typical example of mathematics around us.

We can't live without all kinds of knowledge. Poetry and songs have Chinese, data calculation has mathematics, and songs have music. Therefore, we must study hard and enrich our life with our knowledge and mind.