Put 10 balls of each color red, yellow and blue into a bag. At least how many balls are taken to ensure that two balls of the same color are taken.
把红、黄、蓝三种颜色的球各10个放到一个袋子里．至少取多少个球，可以保证取到两个颜色相同的球．
<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<result>4</result>