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.

# Answer

48
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Some friends went hiking for 3.5 hours. They traveled 21 kilometers in that time. Birgit was 4 minutes/km faster than the average time. If Birgit kept the same pace, how many minutes would it take her to go 8 kilometers?
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
48
Some friends went hiking for 3.5 hours. They traveled 21 kilometers in that time. Birgit was 4 minutes/km faster than the average time. If Birgit kept the same pace, how many minutes would it take her to go 8 kilometers?
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