The father is 48 years old this year, and the son is 21 years old this year. How many years ago, the father was four times the age of the son.
爸爸今年48岁，儿子今年21岁，多少年前爸爸的年龄是儿子的4倍．
<gadget id="calculator">48 - 21</gadget>
<output>27</output>

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

<gadget id="calculator">27 / 3</gadget>
<output>9</output>

<gadget id="calculator">21 - 9</gadget>
<output>12</output>

<result>12</result>