Dad is 30 years old this year, which happens to be three times the age of his daughter. How many years later will the father be twice the age of the daughter?
爸爸今年30岁，恰好是女儿年龄的3倍．多少年后，爸爸的年龄是女儿的2倍？
<gadget id="calculator">30 / 3</gadget>
<output>10</output>

<gadget id="calculator">30 - 10</gadget>
<output>20</output>

<gadget id="calculator">2 - 1</gadget>
<output>1</output>

<gadget id="calculator">20 / 1</gadget>
<output>20</output>

<gadget id="calculator">20 - 10</gadget>
<output>10</output>

<result>10</result>