There are 2 yellow balls in the box. If one ball is randomly drawn each time, how many red balls should be put in to make the probability of drawing a yellow ball 20%.
盒子中有2个黄球，如果每次任意摸出一个球，要使摸出黄球的可能性为20%，还要放入多少个红球．
<gadget id="calculator">20 / 100</gadget>
<output>1/5 = around 0.2</output>

<gadget id="calculator">2 / (1/5)</gadget>
<output>10</output>

<gadget id="calculator">10 - 2</gadget>
<output>8</output>

<result>8</result>