The father is 30 years older than the son this year. Three years later, the father will be four times as old as the son. How old is the son this year?
父亲今年比儿子大30岁，3年后，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">30 / 3</gadget>
<output>10</output>

<gadget id="calculator">10 - 3</gadget>
<output>7</output>

<result>7</result>