The average of the three numbers A, B, and C is 3.5, and the average of the two numbers B and C is 4. What is the number of A?
甲、乙、丙三个数的平均数是3.5，乙丙两个数的平均数是4，甲数是多少．
<gadget id="calculator">3.5 * 3</gadget>
<output>10.5</output>

<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">10.5 - 8</gadget>
<output>2.5</output>

<result>2.5</result>