A pocket contains 10 balls of three different colors, red, yellow, and blue. To find 10 balls of the same color, how many balls must be drawn at most?
一个口袋装有红、黄、蓝三种不同颜色的小球各10个，要摸出10个相同颜色的小球，至多要摸出多少个．
<gadget id="calculator">9 * 3</gadget>
<output>27</output>

<gadget id="calculator">27 + 1</gadget>
<output>28</output>

<result>28</result>