Father and son are 60 years old this year, and 4 years ago, the age of the father was exactly three times that of the son. How old is the son this year?
父亲和儿子今年共有60岁，又知4年前，父亲的年龄正好是儿子的3倍，儿子今年是多少岁？
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">60 - 8</gadget>
<output>52</output>

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

<gadget id="calculator">52 / 4</gadget>
<output>13</output>

<gadget id="calculator">13 + 4</gadget>
<output>17</output>

<result>17</result>