20 black balls and 10 white balls are placed in a cloth bag, at least how many balls are taken out to ensure that there are 5 black balls and 5 white balls.
20个黑球，10个白球装在一个布袋里，至少拿出多少个才能保证有5个黑球，5个白球．
<gadget id="calculator">10 + 5</gadget>
<output>15</output>

<result>15</result>