There are 4 red balls and 5 yellow balls in a box. If you touch a ball randomly, how many yellow balls should you put in to make the probability of touching a red ball be (1/3).
一个盒子里有4个红球和5个黄球，从中任意摸一个球，要使摸到红球的可能性是(1/3)，还要放入多少个黄球．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">4 / (1/3)</gadget>
<output>12</output>

<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">12 - 9</gadget>
<output>3</output>

<result>3</result>