total of 20 red and yellow balls are placed in the pocket, so that the probability of touching the red ball is (1/4), how many yellow balls should be placed.
在口袋里共放20个红球和黄球，要让摸到红球的可能性是(1/4)，应放多少个黄球．
<gadget id="calculator">1 / 4</gadget>
<output>1/4 = around 0.25</output>

<gadget id="calculator">1 - (1/4)</gadget>
<output>3/4 = around 0.75</output>

<gadget id="calculator">20 * (3/4)</gadget>
<output>15</output>

<result>15</result>