He covers 4km/hr*4hrs = <<4*4=16>>16km in the first 4 hours
He has 24-16 = <<24-16=8>>8km left to cover
He has 8-4 = <<8-4=4>>4 hours to cover the remaining 8km
He has to travel at a speed of 8km/4hrs = 2km/hr to be right on time
#### 2