Xiao Min's mother can only bake two pancakes at a time, and both sides need to be baked, each side takes 3 minutes, how many minutes does it take at least to bake 7 pancakes?
小敏的妈妈烙饼，每次只能烙两张饼，两面都要烙，每面3分钟，烙7张饼至少需要多少分钟．
<gadget id="calculator">4 * 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>