Each notebook is 4 yuan, buy 5 and get 1 free. Now we need 6 notebooks, how much money is enough. how much
每个笔记本4元，买5本送1本．现在需要6个笔记本，用多少钱就够了．多少元
<gadget id="calculator">4 * 5</gadget>
<output>20</output>

<result>20</result>