4 years ago, the age of the father and son was 40 years old. This year, the age of the father is three times that of the son. How old is the son this year?
4年前父子年龄和是40岁，今年父亲年龄是儿子的3倍，今年儿子多少岁？
<gadget id="calculator">40 + 4 + 4</gadget>
<output>48</output>

<gadget id="calculator">3 + 1</gadget>
<output>4</output>

<gadget id="calculator">48 / 4</gadget>
<output>12</output>

<result>12</result>