On Monday and Wednesday, Rosie runs 6*1= <<6*1=6>>6 miles.
On Tuesday, Rosie runs 6*(30 minutes/60 minutes) = <<6*(30/60)=3>>3 miles.
On Thursday, Rosie runs 6*(20 minutes/60 minutes)=<<6*(20/60)=2>>2 miles.
So far this week Rosie has run 6+3+6+2=<<6+3+6+2=17>>17 miles.
To reach her goal, she must run 20-17=<<20-17=3>>3 miles on Friday.
Thus, she should run for 6/3=.5 hour which is 30 minutes.
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