There are two kinds of red and white balls in the box. Now there are 3 white balls. To make the probability that a ball is a white ball randomly drawn is (1/3), how many red balls should be placed.
在箱子里放红白两种球，现放了白球3个，要使随意摸出一个球是白球的可能性是(1/3)，还应放红球多少个．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">3 / (1/3)</gadget>
<output>9</output>

<gadget id="calculator">9 - 3</gadget>
<output>6</output>

<result>6</result>