Each copy of "Story King" is 12 yuan, buy 2 copies and get one free. How much is it cheaper to buy 2 copies at a time?
每本《故事大王》12元，买2本送一本．一次买2本，每本便宜多少元钱？
<gadget id="calculator">2 + 1</gadget>
<output>3</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">12 * (2/3)</gadget>
<output>8</output>

<gadget id="calculator">12 - 8</gadget>
<output>4</output>

<result>4</result>