Xiao Lin is 12 years old this year, and his father is 42 years old this year. How many years later, his father's age is three times that of Xiao Lin.
小林今年12岁，爸爸今年42岁，再过多少年爸爸的年龄是小林的3倍．
<gadget id="calculator">42 - 12</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 - 12</gadget>
<output>3</output>

<result>3</result>