The age of the father 15 years ago is equivalent to the age of the son 12 years later. When the age of the father is four times that of the son, how old is the father?
爸爸15年前的年龄相当于儿子12年后的年龄，当爸爸年龄是儿子的4倍时，爸爸多少岁．
<gadget id="calculator">15 + 12</gadget>
<output>27</output>

<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">27 / 3</gadget>
<output>9</output>

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

<result>36</result>