This year the age of the father is 4 times the age of the son, 20 years later, the age of the father is twice the age of the son, how old is the father this year?
今年爸爸的年龄是儿子的4倍，20年后，爸爸的年龄是儿子的2倍，爸爸今年多大？
<gadget id="calculator">20 * 2</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>

<gadget id="calculator">10 * 4</gadget>
<output>40</output>

<result>40</result>