My mother is 40 years old this year, which happens to be four times Xiao Ming's age. How many years later, my mother's age is twice Xiao Ming's.
妈妈今年40岁，恰好是小明年龄的4倍，多少年后妈妈的年龄是小明的2倍．
<gadget id="calculator">40 / 4</gadget>
<output>10</output>

<gadget id="calculator">40 - 10</gadget>
<output>30</output>

<gadget id="calculator">30 * 2</gadget>
<output>60</output>

<gadget id="calculator">60 - 40</gadget>
<output>20</output>

<result>20</result>