The sum of the ages of the elder sister and the younger sister after two years is 25. This year, the age of the younger sister is only half of that of the older sister. How old is the younger sister this year?
姐姐和妹妹两年后的年龄之和是25，今年妹妹的年龄只有姐姐的一半，妹妹今年几岁？
<gadget id="calculator">25 - 4</gadget>
<output>21</output>

<gadget id="calculator">1 / 2</gadget>
<output>1/2 = around 0.5</output>

<gadget id="calculator">1 + (1/2)</gadget>
<output>3/2 = around 1.5</output>

<gadget id="calculator">21 / (3/2)</gadget>
<output>14</output>

<gadget id="calculator">14 * (1/2)</gadget>
<output>7</output>

<result>7</result>