Xiaoli saves money and wants to buy a set of 4 books in the "Encyclopedia of Knowledge" series, the book price is 23.2 yuan. Xiaoli saved enough money to go to the bookstore to buy books, just in time for the bookstore to promote the "Encyclopedia of Knowledge" series for only 17.4 yuan. Xiaoli bought two notebooks with the remaining money. How much cheaper per book on average?
小丽攒钱想买一套4本的《百科知识》丛书，书价是23.2元．小丽攒够了钱去书店买书，刚好碰上书店促销《百科知识》丛书只售17.4元．小丽就用剩下的钱买了两本笔记本．平均每本书便宜了多少钱？
<gadget id="calculator">23.2 - 17.4</gadget>
<output>5.8</output>

<gadget id="calculator">5.8 / 4</gadget>
<output>1.45</output>

<result>1.45</result>