commodity is 25 yuan per piece. Buy 4 pieces and get one free during the New Year's Day. If you buy 4 pieces at a time, how much is each piece cheaper?
一种商品每件25元，元旦期间买4件送一件，一次买4件，每件便宜多少元钱．
<gadget id="calculator">25 * 4</gadget>
<output>100</output>

<gadget id="calculator">100 / 5</gadget>
<output>20</output>

<gadget id="calculator">25 - 20</gadget>
<output>5</output>

<result>5</result>