The store conducts a reward activity, each pen is 20 yuan, buy 4 pens and get 1 free, if you buy 4 pens at a time, how much is each pen cheaper?
商店进行酬宾活动，每支钢笔20元，买4支送1支，如果一次买4支，每支便宜多少钱？
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">20 * (4/5)</gadget>
<output>16</output>

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

<result>4</result>