The average of the three numbers A, B, and C is 17.07, and the average of the two numbers A and C is 10.35. What is the number of B?
甲、乙、丙三个数的平均数是17.07，甲丙两数的平均数是10.35，乙数是多少．
<gadget id="calculator">17.07 * 3</gadget>
<output>51.21</output>

<gadget id="calculator">10.35 * 2</gadget>
<output>20.7</output>

<gadget id="calculator">51.21 - 20.7</gadget>
<output>30.51</output>

<result>30.51</result>