The average age of Dongdong, father, and mother this year is 30 years old. It is known that the average age of father and mother is 39 years old. How old do you think Dongdong is this year?
东东、爸爸、妈妈三人今年的平均年龄是30岁，已知爸爸和妈妈两人的平均年龄是39岁，你猜东东今年是多少岁？
<gadget id="calculator">30 * 3</gadget>
<output>90</output>

<gadget id="calculator">39 * 2</gadget>
<output>78</output>

<gadget id="calculator">90 - 78</gadget>
<output>12</output>

<result>12</result>