On New Year's Day, the store held an activity, each towel was 12 yuan, buy 3 get 1 free, mother bought 3 at a time, how much is each towel cheaper.
元旦节商店搞活动，每根毛巾12元，买3送1，妈妈一次买3根，每根毛巾便宜多少元钱．
<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">36 / 4</gadget>
<output>9</output>

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

<result>3</result>