Xiaohong is 9 years old this year, and her father is 39 years old this year. How many years later, her father's age is exactly twice that of Xiaohong's.
小红今年9岁，爸爸今年39岁，多少年后爸爸的年龄正好是小红年龄的2倍．
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">9 / 1</gadget>
<output>9</output>

<gadget id="calculator">39 - 9 - 9</gadget>
<output>21</output>

<result>21</result>