The father is 41 years old this year, and the son is 13 years old this year. After a few years, the age of the father is exactly twice the age of the son.
父亲今年41岁，儿子今年13岁，过多少年后，父亲的年龄正好是儿子年龄的2倍．
<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">13 / 1</gadget>
<output>13</output>

<gadget id="calculator">41 - 13 - 13</gadget>
<output>15</output>

<result>15</result>