We first convert the time of 4 hours 20 minutes in minutes 2 hours 20 minutes = 2 × 60 + 20 = 140 minutes
Average speed S is given by distance / time. Hence S = 280 miles / 140 minutes = <<280/140=2>>2 mile / minute
There are 60 minutes in an hour so the speed is 2*60=120 miles/hour
#### 120