Minutes spent hiking = 3.5*60 = <<3.5*60=210>>210 minutes
Average speed = 210 minutes/ 21 km = <<210/21=10>>10 minutes/km
Birgit is 4 minutes/km faster = 10 - 4 = 6 min/km
It would take 6*8 = <<6*8=48>>48 minutes to go 8 km
Birgit would take 48 minutes to travel 8 kilometers.
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