Put 18 balls in the pocket, touch a ball at random, so that the possibility of touching a red ball is (5/6), how many red balls should be put in it.
口袋里放18个球，任意摸一个球，要使摸到红球的可能性为(5/6)，其中应放入多少个红球．
<gadget id="calculator">5 / 6</gadget>
<output>5/6 = around 0.833333</output>

<gadget id="calculator">18 * (5/6)</gadget>
<output>15</output>

<result>15</result>