Dongdong is 15 years old this year, and Xixi is 3 years old this year. How many years later, Dongdong will be three times as old as Xixi.
东东今年15岁，西西今年3岁，多少年后东东的年龄是西西的3倍．
<gadget id="calculator">3 * 3</gadget>
<output>9</output>

<gadget id="calculator">15 - 9</gadget>
<output>6</output>

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

<gadget id="calculator">6 / 2</gadget>
<output>3</output>

<result>3</result>