There are 10 red balls, 8 green balls and some black balls in a box, and each time one ball is taken out of it, the probability of taking out a green ball is less than (1/3), then at least how many black balls are there ball.
在一个盒子中有10个红球，8个绿球和一些黑球，每次从里面拿出1个球，结果拿出绿球的可能性小于(1/3)，那么至少有多少个黑球．
<gadget id="calculator">25 - 10 - 8</gadget>
<output>7</output>

<result>7</result>