The father is 27 years older than the son, and after 4 years the father will be 4 times as old as the son. How old is your son?
父亲比儿子大27岁，4年之后父亲的年龄是儿子的4倍．儿子今年多少岁？
<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>5</output>

<result>5</result>