In a family of four, the father is 3 years older than the mother, and the elder brother is 3 years older than the younger sister. 15 years ago, the combined age of their family was 68. Now the combined age of their family is 126. How old is the younger sister?
一家四口人，爸爸比妈妈大3岁，哥哥比妹妹大3岁，15年前他们一家人的年龄之和是68岁，现在他们一家人年龄之和是126岁，妹妹今年多少岁．
<gadget id="calculator">15 * 4</gadget>
<output>60</output>

<gadget id="calculator">68 + 60</gadget>
<output>128</output>

<gadget id="calculator">128 - 126</gadget>
<output>2</output>

<gadget id="calculator">15 - 2</gadget>
<output>13</output>

<result>13</result>