The sum of A and B is 40, the sum of B and C is 30, and the sum of A and C is 50. What is the average of A, B, and C?
甲和乙两数的和为40，乙和丙两数的和为30，甲和丙两数的和为50，求甲、乙、丙三数的平均数是多少．
<gadget id="calculator">40 + 30 + 50</gadget>
<output>120</output>

<gadget id="calculator">120 / 2</gadget>
<output>60</output>

<gadget id="calculator">60 / 3</gadget>
<output>20</output>

<result>20</result>