If you use a pan to cook pancakes, you can cook 2 cakes at the same time. It takes 6 minutes to cook one cake (3 minutes for the front and 3 minutes for the reverse side), and how many minutes is the minimum time to cook 7 cakes.
用一只平底锅煎饼，每次能同时煎2张饼，煎一张饼需要6分钟(煎正、反面各需3分钟)，煎7张饼至少需要多少分钟．
<gadget id="calculator">6 * 2</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>