pen used to be 12 yuan, and the People's Store bought 3 pens and got 1 free for promotion. Teacher Wang bought 3 at a time, how much cheaper can each be on average?
一支钢笔原来12元，人民商店为了促销，买3支送1支．王老师一次买了3支，平均每支可以便宜多少元？
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">3 / 4</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">12 * (3/4)</gadget>
<output>9</output>

<gadget id="calculator">12 - 9</gadget>
<output>3</output>

<result>3</result>