Each disc is 20 yuan, buy 4 and get 1 free. Uncle Wang needs to buy 5 such discs. How much is each disc cheaper?
每张光碟20元，买4张送l张．王叔叔需要买5张这样的光碟，每张光碟便宜了多少元？
<gadget id="calculator">20 * 4</gadget>
<output>80</output>

<gadget id="calculator">80 / 5</gadget>
<output>16</output>

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

<result>4</result>