400
Mary took an hour out of her busy day and drove 55 miles to John's house. John, meanwhile, had driven 75 miles to David's house. Mary called John, they laughed about the miscommunication, and then John and David drove back to John's house. If John drives to David's house at the same speed as Mary, and drives back to his own house at half that speed, how much time has John spent driving today? Round to the nearest hour.
Assuming that the amount of time he took to run out for milk this morning is negligible, approximately 4 hours.