The school organized a spring outing. The students set off from the school at one o'clock in the afternoon, walked a flat road, climbed a mountain and then returned by the same road, and returned to school at seven o'clock in the afternoon. It is known that their walking speed is 4 km/h on the flat road, 3 km/h when climbing the mountain, 6 km/h when going down the mountain, and 2.5 hours when returning. Question: How far did they travel in total?
学校组织春游，同学们下午一点从学校出发，走了一段平路，爬了一座山后按原路返回，下午七点回到学校．已知他们的步行速度平路4千米小时，爬山3千米/小时，下山为6千米/小时，返回时间为2.5时．问：他们一共行了多少路？
<gadget id="calculator">7 - 1 - 3</gadget>
<output>3</output>

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

<gadget id="calculator">1 * 1</gadget>
<output>1</output>

<gadget id="calculator">7 - 1</gadget>
<output>6</output>

<gadget id="calculator">2.5 - 1</gadget>
<output>1.5</output>

<gadget id="calculator">1.5 * 4</gadget>
<output>6</output>

<gadget id="calculator">6 + 6</gadget>
<output>12</output>

<gadget id="calculator">12 * 2</gadget>
<output>24</output>

<result>24</result>