There are 10 single-color balls of four colors: red, yellow, blue, and white, and they are mixed and put into a cloth bag. Then at least how many balls must be drawn to ensure that there are four colors in the drawn balls.
有红、黄、蓝、白四种颜色的单色球各10个，混合后放到一条布袋里．那么至少要摸出多少个球，才能保证摸出的球中四种颜色都有．
<gadget id="calculator">10 * 3</gadget>
<output>30</output>

<gadget id="calculator">30 + 1</gadget>
<output>31</output>

<result>31</result>