The ratio of A and B is 5:2. The average of these two numbers is 4.2. What is A's number?
甲、乙两数的比是5：2，这两个数的平均数得4.2，甲数是多少．
<gadget id="calculator">4.2 * 2</gadget>
<output>8.4</output>

<gadget id="calculator">5 / 7</gadget>
<output>5/7 = around 0.714286</output>

<gadget id="calculator">8.4 * (5/7)</gadget>
<output>6</output>

<result>6</result>