The age of the elder brother 7 years ago was 10 years old and the age of the younger sister 5 years later is the same. When the elder brother is 10 years old, it is exactly three times the age of the younger sister.
哥哥7年前的年龄是10岁和妹妹5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">7 + 5</gadget>
<output>12</output>

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

<gadget id="calculator">12 / 2</gadget>
<output>6</output>

<gadget id="calculator">6 * 3</gadget>
<output>18</output>

<result>18</result>