The school carried out the "walking growth" activity. The team walked 18 kilometers from the school to the base in the suburbs. From the school to the base and back to the school, it took 7 (2/5) hours on the road. From the school to the base, we walked for a while On an uphill road, there is a section of flat road in the middle, and then a section of downhill road. If the walking speed of the team is 4, 5, and 6 kilometers per hour on the uphill, flat road, and downhill respectively, find the distance in kilometers of the middle section of the flat road ?
学校开展"徒步成长"活动，队伍从学校步行到城郊基地有18千米的路程，从学校出发到基地在返回学校，路上共用了7(2/5)小时，从学校到基地，先走一段上坡路，中间是一段平路，然后是一段下坡路，如果队伍行走的速度在上坡、平路、下坡分别是每小时4、5、6千米，求中间一段平路的路程是多少千米？
<gadget id="calculator">18 - 12</gadget>
<output>6</output>

<result>6</result>