I was 8 years old when my father was 31. My father's grade is twice as old as mine this year. How old should I be this year?
我父亲31岁时我8岁，今年父亲的年级比我大一倍，我今年应该几岁？
<gadget id="calculator">31 - 8</gadget>
<output>23</output>

<result>23</result>