The sum of the ages of the elder sister and younger sister three years later is 33 years old. The age of the younger sister this year is equal to the age difference between the two. How old is the elder sister this year?
姐姐与妹妹3年后的年龄和是33岁，妹妹今年的年龄等于两人的年龄差，姐姐今年多少岁．
<gadget id="calculator">3 * 2</gadget>
<output>6</output>

<gadget id="calculator">33 - 6</gadget>
<output>27</output>

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

<gadget id="calculator">27 / 3</gadget>
<output>9</output>

<gadget id="calculator">27 - 9</gadget>
<output>18</output>

<result>18</result>