Xiaoli is 8 years old this year, and Huanhuan is 7 years old this year. When their age sum is 21, how old is Xiaoli?
小立今年8岁，欢欢今年7岁，当他们的年龄和为21岁时，小立是多少岁．
<gadget id="calculator">8 - 7</gadget>
<output>1</output>

<gadget id="calculator">21 + 1</gadget>
<output>22</output>

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

<result>11</result>