A fourth-grade mathematics "Standard Training" is 20 yuan, buy 3 copies and get 1 free. Xiaohong bought 3 copies at a time. How much is each cheaper?
一本四年级数学《达标训练》20元，买3本送1本．小红一次买了3本．每本便宜多少钱？
<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">60 / 4</gadget>
<output>15</output>

<gadget id="calculator">20 - 15</gadget>
<output>5</output>

<result>5</result>