The father is 42 years old this year, and the son is 12 years old this year. How many years later, the father's age will be three times the son's age.
爸爸今年42岁，儿子今年12岁，多少年后爸爸的年龄是儿子年龄的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>