Nini has a bag, which contains 3 black glass balls, 6 golden glass balls, 2 purple glass balls, and 6 red glass balls. Nini added some white glass balls to the bag, and told Yingying that if she randomly takes a ball, the probability of getting black and gold is (3/7). Nini added a few white balls to the bag.
妮妮有一个袋子，袋子中装有3个黑色的玻璃球，6个金色的玻璃球，2个紫色的玻璃球，6个红色的玻璃球．妮妮又向袋子中加了一些白色的玻璃球，并且告诉迎迎她若任取一个球得到黑色和金色的概率是(3/7)．妮妮向袋子中加了多少个白色的球．
<gadget id="calculator">6 + 3</gadget>
<output>9</output>

<gadget id="calculator">3 / 7</gadget>
<output>3/7 = around 0.428571</output>

<gadget id="calculator">9 / (3/7)</gadget>
<output>21</output>

<gadget id="calculator">21 - 3 - 6 - 2 - 6</gadget>
<output>4</output>

<result>4</result>