Peter has to walk a remaining 2.5 total miles - 1 mile completed = 1.5 miles remaining
It will take him another 20 minutes * 1.5 miles remaining = 30 minutes to get to the grocery store
Peter has to walk 2.5 miles to get to the grocery store. If it takes him 20 minutes to walk one mile and he has walked 1 mile already, how many more minutes does he have to walk to reach it?