A basket of apples, 3 numbers of 3, and 7 numbers of 7 are exactly counted. How many apples are there at least in this basket?
一筐苹果，3个3个的数，7个7个的数都正好数完，这筐苹果最少有多少个？
<gadget id="calculator">3 * 7</gadget>
<output>21</output>

<result>21</result>