The average of the four numbers A, B, C, and D is 6, and the average of the three numbers B, C, and D is 7. What is A?
A、B、C、D四个数的平均数是6，B、C、D三个数的平均数是7，A是多少？
<gadget id="calculator">6 * 4</gadget>
<output>24</output>

<gadget id="calculator">7 * 3</gadget>
<output>21</output>

<gadget id="calculator">24 - 21</gadget>
<output>3</output>

<result>3</result>