There are 84 red balloons, the ratio of red balloons to yellow balloons is 7:5, how many yellow balloons are there, (use the knowledge of proportion to answer)
有84个红气球，其中红气球和黄气球的比是7：5，黄气球有多少个，(用比例的知识解答)
<gadget id="calculator">84 * 5</gadget>
<output>420</output>

<gadget id="calculator">420 / 7</gadget>
<output>60</output>

<result>60</result>