The average of the three numbers A, B, and C is 10, the average of A and B is 11, and the sum of B and C is 19. What is the number of A?
甲、乙、丙三个数的平均数是10，甲、乙平均数为11，乙、丙之和为19，甲数是多少．
<gadget id="calculator">10 * 3</gadget>
<output>30</output>

<gadget id="calculator">30 - 19</gadget>
<output>11</output>

<result>11</result>