The New Year's calendar is 15 yuan each, buy 4 and get 1 free, Aunt Zhang buys 4 at a time, how much is each cheaper?
新年的挂历每本15元，买4本送1本，张阿姨一次买4本，每本便宜多少钱？
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">60 / 5</gadget>
<output>12</output>

<gadget id="calculator">15 - 12</gadget>
<output>3</output>

<result>3</result>