Use a pan to bake pancakes, you can only put 2 cakes at a time, it takes 3 minutes to bake one side, and both sides have to be baked, how many minutes does it take at least to bake 7 cakes.
用一只平底锅烙饼，每次只能放2张饼，烙一面要3分钟，两面都要烙，烙7张饼至少要用多少分钟．
<gadget id="calculator">2 * 2 * 3</gadget>
<output>12</output>

<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">12 + 9</gadget>
<output>21</output>

<result>21</result>