There are 7 balls of the same shape and size in an opaque box, 2 red balls and 5 yellow balls. If a ball is randomly drawn from it, what is the probability that it is exactly a red ball?
在一个不透明的盒子里放有7个形状大小相同的球，2个红球，5个黄球，从中任意取出一个球，正好是红球的可能性是多少
<gadget id="calculator">5 + 2</gadget>
<output>7</output>

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

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