Each calendar is 18 yuan, buy 5 and get 1 free. Auntie bought 5 copies, how much is each cheaper.
挂历每本18元，买5本送1本．阿姨买了5本，每本便宜多少元．
<gadget id="calculator">5 / 6</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">18 * (5/6)</gadget>
<output>15</output>

<gadget id="calculator">18 - 15</gadget>
<output>3</output>

<result>3</result>