Teacher Wang brought 500 yuan, bought 24 books, and left 20 yuan. How much is the average book?
王老师带了500元钱，买了24本书，剩20元钱．平均每本书多少钱？
<gadget id="calculator">500 - 20</gadget>
<output>480</output>

<gadget id="calculator">480 / 24</gadget>
<output>20</output>

<result>20</result>