Mom uses a pan to bake pancakes. It takes 3 minutes to bake one side of the cake. You can put up to 2 cakes in the pan at a time. If you want to bake 7 cakes, how many minutes will it take at least?
妈妈用平底锅烙饼，烙一个饼的一面用3分钟，锅中每次最多能放2张饼，要烙7张饼，至少需要多少分钟？
<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>