China Resources Suguo held a sales promotion on weekends. Each bag of washing powder was 12 yuan. Buy three and get one free. Mom bought 3 bags at a time. How much is each bag cheaper than usual?
华润苏果周末搞促销活动，洗衣粉每袋12元，买三送一，妈妈一次买了3袋，每袋比平时便宜多少元？
<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>