The sum of A and B is 35, the sum of B and C is 27, and the sum of A and C is 28. What is the average of the three numbers A, B, and C?
甲、乙两数的和是35，乙、丙两数的和是27，甲、丙两数的和是28．甲、乙、丙三个数的平均数是多少？
<gadget id="calculator">35 + 27 + 28</gadget>
<output>90</output>

<gadget id="calculator">90 / 2 / 3</gadget>
<output>15</output>

<result>15</result>