There are 10 red, yellow and green balls of the same shape. Mix them into the same bag. At least touch a few balls at a time to ensure a pair of balls of the same color with two colors?
有外形相同的红、黄、绿三色球各10个．混合放入同一布袋中．一次至少摸几个球，才能保证有两种颜的同色球各一对？
<gadget id="calculator">10 + 2 + 1</gadget>
<output>13</output>

<result>13</result>