In a box containing 4 red balls and 5 black balls, what is the probability of drawing a red ball?
在一个装有4个红球，5个黑球的盒子里，摸到红球的可能性是多少．
<gadget id="calculator">4 + 5</gadget>
<output>9</output>

<gadget id="calculator">4 / 9</gadget>
<output>4/9 = around 0.444444</output>

<result>4/9 = around 0.444444</result>