Put 9 balls in the pocket, touch a ball at random, so that the probability of touching a red ball is (2/3), how many red balls should be put in.
在口袋里放入9个球，任意摸一个球，要使摸到红球的可能性是(2/3)，要放入多少个红球．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">9 * (2/3)</gadget>
<output>6</output>

<result>6</result>