A pan can only bake 2 cakes at a time, and each side takes 3 minutes. How long does it take to cook 7 cakes at least?
一个平底锅每次只能烙2张饼，每面3分钟，烙熟7张饼至少需要多少分钟．
<gadget id="calculator">6 * 2</gadget>
<output>12</output>

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

<result>21</result>