The sum of the ages of grandpa and dad this year is exactly 80 years old. Four years ago, grandpa's age was exactly twice that of father's. How old is father this year?
今年爷爷和爸爸的年龄和正好是80岁．4年前，爷爷的年龄正好是爸爸的2倍，爸爸今年多少岁．
<gadget id="calculator">4 * 2</gadget>
<output>8</output>

<gadget id="calculator">80 - 8</gadget>
<output>72</output>

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

<gadget id="calculator">72 / 3</gadget>
<output>24</output>

<gadget id="calculator">24 + 4</gadget>
<output>28</output>

<result>28</result>