In a box with 5 yellow balls and 2 white balls, if you touch a ball randomly, the probability of getting a yellow ball is ((())/(())), and the probability of getting a black ball is (1/2), how many black balls to add.
在装有5个黄球，2个白球的盒子中，任意摸一个球，摸到黄球的可能性是((())/(()))，要使摸到黑球的可能性是(1/2)，要加进多少个黑球．
<gadget id="calculator">5 + 2</gadget>
<output>7</output>

<result>7</result>