A box contains 1 red ball, 1 green ball and 2 white balls. What is the probability that both balls are white if they are all the same except for their color and two balls are taken from the box at a time?
一个盒子里有1个红球，1个绿球，2个白球．除颜色外其余都相同，现在从盒子里一次取出两个球，则这两个球都是白球的可能性是多少．
<gadget id="calculator">1 / 6</gadget>
<output>1/6 = around 0.166667</output>

<result>1/6 = around 0.166667</result>