The age of the father is 4 times that of the son this year, and the age of the father is 2 times that of the son in 20 years. How old is your son this year?
今年父亲的年龄是儿子的4倍，20年后父亲的年龄是儿子的2倍．今年儿子多少岁？
<gadget id="calculator">2 * 20</gadget>
<output>40</output>

<gadget id="calculator">40 - 20</gadget>
<output>20</output>

<gadget id="calculator">4 - 2</gadget>
<output>2</output>

<gadget id="calculator">20 / 2</gadget>
<output>10</output>

<result>10</result>