There are 15 red, yellow and blue balls in the same shape in each bag. Each child draws 2 small balls from it. At least how many children touch the ball to ensure that two people touch the same ball.
袋中有外形完全一样的红、黄、蓝三种颜色的小球各15个．每个小朋友从中摸出2个小球．至少有多少个小朋友摸球，才能保证一定有两个人摸的球一样．
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">6 + 1</gadget>
<output>7</output>

<result>7</result>