The father is 28 years older than the son, and the age of the father is 1 year more than four times the age of the son. How old is the son this year?
父亲比儿子大28岁，父亲的年龄比儿子年龄的4倍多1岁，儿子今年多少岁．
<gadget id="calculator">28 - 1</gadget>
<output>27</output>

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

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

<result>9</result>