The average age of Wang Huan, father, and mother this year is exactly 30 years old. It is known that the average age of father and mother is 39 years old. How old is Wang Huan this year?
王欢、爸爸、妈妈今年三人的平均年龄正好是30岁，已知爸爸妈妈两人的平均年龄是39岁，王欢今年是多少岁．
<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>