Johnny spends 6/3=2 hours walking the 6 dogs who have 60-minute walks.
He makes $20*6=$<<20*6=120>>120 for these walks.
Thus, he has 4-2=2 hours=120 minutes left to walk the other dogs.
He can walk 3*(120/30)=<<3*(120/30)=12>>12 dogs in this time.
Thus, he will make $15*12=$<<12*15=180>>180 on these walks.
In total per day, he makes $120+$180=$<<120+180=300>>300 per day.
Thus, during the week, he makes $300*5=$<<300*5=1500>>1500.
#### 1500