The average of the three numbers A, B, and C is 20, the average of the two numbers A, B is 15, what is the number of C
甲、乙、丙三个数的平均数是20，甲、乙两个数的平均数是15，丙数是多少
<gadget id="calculator">20 * 3</gadget>
<output>60</output>

<gadget id="calculator">15 * 2</gadget>
<output>30</output>

<gadget id="calculator">60 - 30</gadget>
<output>30</output>

<result>30</result>