David drove 70 miles in the first 1 ¾ hours of his trip. How much longer should it take him to drive an additional 120 miles if he drives at the same average speed?
To find the speed at which David is driving, we divide the distance he drove by the time it took him to drive that distance: 70 miles / (7/4) hours = 40 miles/hour
To find how long it takes him to drive an additional 120 miles at that speed, we divide the distance by the speed: 120 miles / 40 miles/hour = <<120/40=3>>3 hours. Answer: \boxed{3}.