The age of the elder brother 5 years ago is the same as the age of the younger sister 3 years later, when the age of the elder brother is exactly three times the age of the younger sister.
哥哥5年前的年龄和妹妹3年后的年龄相等，当哥哥多少岁时，正好是妹妹年龄的3倍．
<gadget id="calculator">5 + 3</gadget>
<output>8</output>

<gadget id="calculator">3 - 1</gadget>
<output>2</output>

<gadget id="calculator">8 / 2</gadget>
<output>4</output>

<gadget id="calculator">4 + 8</gadget>
<output>12</output>

<result>12</result>