There are 3 white balls and 4 red balls of the same size except for the color in a box, if you touch any one, what is the probability of getting a red ball?
一个盒子里有除颜色外大小相同的3个白球和4个红球，任意摸一个，摸到红球的可能性是多少．
<gadget id="calculator">3 + 4</gadget>
<output>7</output>

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

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