an eight-kilometre walk ,fifteen minutes' walk,10 minutes' w

an eight-kilometre walk ,fifteen minutes' walk,10 minutes' walk,为什么有的前有冠词,有的没有冠词
1.-----How far is it from our school to the seaside?
-----It is an eight-kilometre walk from here.
2.It's only fifteen minutes' walk.But for an 80-year-old woman,it's take her about
thirty minutes.
3.Let's go there on foot.It's only 10 minutes' walk.
4.We will go for an 8-day trip abroad.Will you join us?
请问an eight-kilometre walk ,fifteen minutes' walk,10 minutes' walk,an 8-day trip,为什么有的前有冠词,有的没有冠词.