There are 4 yellow balls, 2 red balls, and 1 black ball in a box. If you touch any ball, what is the probability of getting a yellow ball?
盒子里有4个黄球，2个红球，1个黑球，任意摸一个球，摸到黄球的可能性是多少．
<gadget id="calculator">4 + 2 + 1</gadget>
<output>7</output>

<gadget id="calculator">4 / 7</gadget>
<output>4/7 = around 0.571429</output>

<result>4/7 = around 0.571429</result>