There are 20 red and yellow balls of the same size in a box. Draw a ball at random, so that the probability of drawing a red ball is (2/5), how many red balls should be placed in the box.
盒子里有大小相同的20个红、黄两种球．任意摸出一个球，要使摸到红球的可能性是(2/5)，盒子里应放多少个红球．
<gadget id="calculator">2 / 5</gadget>
<output>2/5 = around 0.4</output>

<gadget id="calculator">20 * (2/5)</gadget>
<output>8</output>

<result>8</result>