We must first convert minutes to hours, so 30 minutes * (1 hour/60 minutes) = <<30*(1/60)=0.5>>0.5 hours
The number of miles Greg travels on his trip home is 0.5 hours * 20 mph = <<0.5*20=10>>10 miles
The total miles Greg travels is 10 + 30 = <<10+30=40>>40 miles
#### 40