There are 1 white ball and several yellow balls in a box, how many yellow balls should be filled in order to make the probability that any ball is white is (1/5).
在一个盒子里装有1个白球和几个黄球，要使任意摸出一个球是白球的可能性是(1/5)，应装多少个黄球．
<gadget id="calculator">1 / 5</gadget>
<output>1/5 = around 0.2</output>

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

<gadget id="calculator">5 - 1</gadget>
<output>4</output>

<result>4</result>