"Andersen's Fairy Tales" is 31 yuan each, and Mr. Wang buys 20 copies at a time. How much will it cost? (Catch up with the June 1 discount, buy 4 copies and get 1 free)
《安徒生童话》每本31元，王老师一次买20本，要花多少钱？(赶上六一优惠，买4本送1本)
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">20 / 5</gadget>
<output>4</output>

<gadget id="calculator">20 - 4</gadget>
<output>16</output>

<gadget id="calculator">31 * 16</gadget>
<output>496</output>

<result>496</result>