Mr. Zhang buys 4 books with 100 yuan and gets back 20 yuan. How much is each book on average?
张老师用100元买4本书，找回20元，平均每本书多少钱？
<gadget id="calculator">100 - 20</gadget>
<output>80</output>

<gadget id="calculator">80 / 4</gadget>
<output>20</output>

<result>20</result>