The father is 43 years old this year, and the son is 13 years old this year. How many years ago, the age of the father was four times the age of the son.
父亲今年43岁，儿子今年13岁，多少年前父亲的年龄是儿子年龄的4倍．
<gadget id="calculator">43 - 13</gadget>
<output>30</output>

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

<gadget id="calculator">30 / 3</gadget>
<output>10</output>

<gadget id="calculator">13 - 10</gadget>
<output>3</output>

<result>3</result>