The age of the two sisters this year is 39 years old. After 3 years, the elder sister is 3 years older than the younger sister. How old is the younger sister this year?
今年姐妹俩的年龄和是39岁，3年后姐姐比妹妹大3岁，今年妹妹多少岁．
<gadget id="calculator">39 - 3</gadget>
<output>36</output>

<gadget id="calculator">36 / 2</gadget>
<output>18</output>

<result>18</result>