Two kinds of balls, red and black, are put into a pocket. They have exactly the same shape and size, and there are 12 black balls. The probability of drawing a red ball is (1/3). How many red balls should be placed in the pocket.
在一个口袋里入红黑两种球，它们的形状大小完全一样，其中黑球12个，要使摸出红球的可能性为(1/3)．口袋里应放红球多少个个．
<gadget id="calculator">1 / 3</gadget>
<output>1/3 = around 0.333333</output>

<gadget id="calculator">1 - (1/3)</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">12 / (2/3)</gadget>
<output>18</output>

<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>