Dongdong is 10 years old this year, and his father is 40 years old this year. When Dongdong is old, his father's age is exactly twice that of Dongdong.
冬冬今年10岁，爸爸今年40岁，冬冬多少岁时，爸爸的年龄正好是冬冬的2倍．
<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<result>30</result>