Xiaogang is 13 years old this year, and his father is three times his age. How old is Xiaogang younger than his father?
小刚今年13岁，爸爸的年龄是他的3倍．小刚比爸爸小多少岁？
<gadget id="calculator">13 * 3</gadget>
<output>39</output>

<gadget id="calculator">39 - 13</gadget>
<output>26</output>

<result>26</result>