The average of the two numbers of A and B is 8.4, the ratio of the two numbers of A and B is 5:3, what is the number of A?
甲乙两个数的平均数是8.4，甲乙两个数的比是5：3，甲数是多少．
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">5 / 8</gadget>
<output>5/8 = around 0.625</output>

<gadget id="calculator">8.4 * 2 * (5/8)</gadget>
<output>10.5</output>

<result>10.5</result>