The ratio of the number A to the sum of the numbers B and C is 2:7, then the number A is the average of the three numbers A, B and C.
甲数与乙、丙两个数和的比是2：7，那么甲数是甲、乙、丙三个数的平均数的多少．
<gadget id="calculator">2 + 7</gadget>
<output>9</output>

<gadget id="calculator">9 / 3</gadget>
<output>3</output>

<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<result>2/3 = around 0.666667</result>