Put 9 balls in a box, draw one at random, so that the probability of touching a red ball is (2/3), how many red balls should be placed.
在盒子里放9个球，任意摸出一个，要使摸到红球的可能性为(2/3)，那么要放多少个红球．
<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>