The first phase of the Nanjing Metro project is divided into two parts: the elevated line and the underground line. The elevated line is about 6.5 kilometers long, and the underground line is 1.6 times longer than the elevated line. How many kilometers longer is the underground line than the elevated line?
南京地铁一期工程分高架线和地下线两部分，其中高架线长约6.5千米，地下线是高架线的1.6倍，地下线比高架线长多少千米？
<gadget id="calculator">6.5 * 1.6</gadget>
<output>10.4</output>

<gadget id="calculator">10.4 - 6.5</gadget>
<output>3.9</output>

<result>3.9</result>