There are four people A, B, C and D, the average age of every three plus the age of the remaining one is 29, 23, 21 and 17 respectively. What is the difference between the oldest and youngest age among these four people?
甲、乙、丙、丁四人，每三个人的平均年龄加上余下一人的年龄分别为29、23、21和17．这四人中最大年龄与最小年龄的差是多少．
<gadget id="calculator">2 / 3</gadget>
<output>2/3 = around 0.666667</output>

<gadget id="calculator">12 / (2/3)</gadget>
<output>18</output>

<result>18</result>