Three years ago, the age of the mother was four times that of the son. The son is 12 years old this year. How old is the mother.
3年前，妈妈的年龄是儿子的4倍，儿子今年12岁，妈妈今年多少岁．
<gadget id="calculator">12 - 3</gadget>
<output>9</output>

<gadget id="calculator">9 * 4</gadget>
<output>36</output>

<gadget id="calculator">36 + 3</gadget>
<output>39</output>

<result>39</result>