Each pack of washing powder is 12 yuan, buy 3 packs and get 1 pack free, buy 3 packs at a time, how much is each pack cheaper?
每包洗衣粉12元，买3包送1包，一次买3包，每包便宜多少钱？
<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>