During the New Year's Day, the Spring Festival Supermarket launched a promotional activity. A notebook costs 6 yuan, buy 4 copies and get 1 free. Obviously buy back 10 copies, which is much cheaper.
元旦期间，新春超市开展促销活动，一个笔记本6元钱，买4本送1本，明明买回10本，共便宜多少．
<gadget id="calculator">4 + 1</gadget>
<output>5</output>

<gadget id="calculator">10 / 5</gadget>
<output>2</output>

<gadget id="calculator">2 * 6</gadget>
<output>12</output>

<result>12</result>