The New Year's calendar is 25 yuan each, buy 4 and get 1 free, and Uncle Liu buys 4 at a time, how much is each cheaper?
新年挂历每本25元，买4本送1本，刘叔叔一次买4本，每本便宜多少钱？
<gadget id="calculator">4 / 5</gadget>
<output>4/5 = around 0.8</output>

<gadget id="calculator">25 * (4/5)</gadget>
<output>20</output>

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

<result>5</result>