This year, the average age of me, father, and mother is exactly 30 years old. It is known that the average age of father and mother is 39 years old. How old am I this year?
今年，我、爸爸、妈妈三人的平均年龄正好是30岁，已知爸爸和妈妈两人的平均年龄是39岁，我今年是多少岁．
<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>