Xiaoyu is 15 years old this year, and Xiaoliang is 12 years old. How many years ago, the sum of the ages of Xiaoyu and Xiaoliang was 15.
今年小宇15岁，小亮12岁，多少年前，小宇和小亮的年龄和是15．
<gadget id="calculator">12 - 15</gadget>
<output>-3</output>

<gadget id="calculator">15 + (-3)</gadget>
<output>12</output>

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<result>6</result>