Column calculation: the sum of the two numbers A and B is 7.8, the sum of the two numbers B and C is 9.2, the sum of the two numbers A and C is 7, what is the average of the three numbers A, B and C?
列式计算：甲乙两数的和是7.8，乙丙两数的和是9.2，甲丙两数的和是7，甲乙丙三个数的平均数是多少？
<gadget id="calculator">7.8 + 9.2 + 7</gadget>
<output>24</output>

<gadget id="calculator">24 / 2</gadget>
<output>12</output>

<gadget id="calculator">12 / 3</gadget>
<output>4</output>

<result>4</result>