The father is 30 years older than the son. Five years ago, the ratio of the age of the father to the age of the son was 4:1. What is the age of the son?
父亲比儿子大30岁，5年前父亲年龄与儿子年龄的比为4：1，儿子的年龄是多少．
<gadget id="calculator">4 - 1</gadget>
<output>3</output>

<gadget id="calculator">30 / 3</gadget>
<output>10</output>

<gadget id="calculator">10 + 5</gadget>
<output>15</output>

<result>15</result>