Xiao Ming is 9 years old this year, and his father is 39 years old this year. How many years later, his father's age is exactly twice Xiao Ming's age.
小明今年9岁，父亲今年39岁，再过多少年父亲年龄正好是小明年龄的2倍．
<gadget id="calculator">9 * 2</gadget>
<output>18</output>

<gadget id="calculator">39 - 18</gadget>
<output>21</output>

<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">21 / 1</gadget>
<output>21</output>

<result>21</result>