There are 4 red balls, 2 yellow balls, and 1 white ball in a bag. If you draw a ball at random, what is the probability of getting a yellow ball?
袋子里有红球4个，黄球2个，白球1个，任意摸出一个球，摸出黄球的可能性是多少．
<gadget id="calculator">4 + 2 + 1</gadget>
<output>7</output>

<gadget id="calculator">2 / 7</gadget>
<output>2/7 = around 0.285714</output>

<result>2/7 = around 0.285714</result>