My younger sister is 6 years old this year, and my older sister is 12 years old. When the sum of their ages is 40 years old, how many years should it be?
今年妹妹6岁，姐姐12岁，当两人的年龄之和是40岁时，应该是多少年之后的事了．
<gadget id="calculator">12 + 6</gadget>
<output>18</output>

<gadget id="calculator">40 - 18</gadget>
<output>22</output>

<gadget id="calculator">22 / 2</gadget>
<output>11</output>

<result>11</result>