It takes 10 minutes to dry the coat of a short-haired dog and twice as long for a full haired dog so they take 10*2 = <<10*2=20>>20 minutes to dry
He had 6 short hair dogs and they took 10 minutes to dry each so they took 6*10= <<6*10=60>>60 minutes to dry
He had 9 full haired dogs that took 20 minutes to dry each so they took 9*20 = <<9*20=180>>180 minutes to dry
Together is takes him 60+180 = <<60+180=240>>240 minutes to dry all of the dogs
60 minutes are in 1 hour and he spends 240 minutes on the dogs so he worked for 240/60 = <<240/60=4>>4 hours drying the dogs
#### 4