In a pocket there are 10 black balls, 10 white balls, and 10 red balls. Question: How many balls must be taken from it at least to ensure that there are white balls in it?
在一个口袋中有10个黑球、10个白球、10个红球．问：至少从中取出多少个球，才能保证其中有白球？
<gadget id="calculator">10 + 10 + 1</gadget>
<output>21</output>

<result>21</result>