30 students participate in a mathematics competition. It is known that there is at least one boy among any 10 participants, so how many boys are there at least?
30名学生参加数学竞赛，已知参赛者中任何10人里都至少有一名男生，那么男生至少有多少人．
<gadget id="calculator">10 - 1</gadget>
<output>9</output>

<gadget id="calculator">30 - 9</gadget>
<output>21</output>

<result>21</result>