Xiaogang is 12 years old this year, and his mother is 40 years old this year. How many years later, his mother's age will be exactly three times that of Xiaogang.
小刚今年12岁，妈妈今年40岁，多少年后妈妈的年龄正好是小刚的3倍．
<gadget id="calculator">12 * 3</gadget>
<output>36</output>

<gadget id="calculator">40 - 36</gadget>
<output>4</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">4 / 2</gadget>
<output>2</output>

<result>2</result>