There are 3 red balls, 4 white balls, and some black balls in a box. After shaking well, the probability of randomly drawing out a ball is white is (4/15), then how many black balls are there in the box.
一个盒子中装有3个红球，4个白球，和一些黑球，摇匀后，任意摸出一个球是白球的可能性是(4/15)，那么这个盒子中有黑球多少个．
<gadget id="calculator">4 / 15</gadget>
<output>4/15 = around 0.266667</output>

<gadget id="calculator">4 / (4/15)</gadget>
<output>15</output>

<gadget id="calculator">3 + 4</gadget>
<output>7</output>

<gadget id="calculator">15 - 7</gadget>
<output>8</output>

<result>8</result>