Teacher Wang bought 3 books for the class, paid the salesperson 100 yuan, and got 7 yuan back. How much is the average price per book?
王老师为班上买回了3本图书，付给营业员100元，找回7元钱．平均每本书多少元？
<gadget id="calculator">100 - 7</gadget>
<output>93</output>

<gadget id="calculator">93 / 3</gadget>
<output>31</output>

<result>31</result>