A box contains 10 red balls, 10 white balls and 10 blue balls of the same size. There must be 2 balls of the same color if you want to draw out, at least how many balls must be drawn out.
盒子里有同样大小的红球、白球和蓝球各10个．要想摸出的球一定有2个同色的，至少要摸出多少个球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>