box of apples is 3 by 3, 7 by 7, and all of them are finished. What is the minimum number of apples in this basket?
一箱苹果3个3个拿，7个7个拿，都正好拿完，这筐苹果最少有多少个．
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<result>21</result>