The father is 50 years old this year, and the son is 20 years old this year. How many years ago, the age of the father was 4 times the age of the son.
父亲今年50岁，儿子今年20岁，多少年前父亲的年龄是儿子年龄的4倍．
<gadget id="calculator">50 - 20</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">20 - 10</gadget>
<output>10</output>

<result>10</result>