Xiao Ming is 9 years old this year, and his father is 39 years old. After many years, his father's age will be exactly three times that of Xiao Ming.
小明今年9岁，父亲39岁，再过多少年，父亲的年龄正好是小明的3倍．
<gadget id="calculator">39 - 9</gadget>
<output>30</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">30 / 2</gadget>
<output>15</output>

<gadget id="calculator">15 - 9</gadget>
<output>6</output>

<result>6</result>