The average of the two numbers A and B is 30.4, the number of A is 40.2, what is the number of B.
A，B两数的平均数是30.4，A数是40.2，B数是多少．
<gadget id="calculator">30.4 * 2</gadget>
<output>60.8</output>

<gadget id="calculator">60.8 - 40.2</gadget>
<output>20.6</output>

<result>20.6</result>