To find the average speed in miles per hour:
[tex] \frac{275}{4} =68.75[/tex]
So he would need to drive at an average speed of 68.75 miles per hour in order to make it to Des Moines in 4 hours.
If the speed limit is 65, Henry better keep an eye out for Highway Patrol, since he MUST break the speed limit in order to average 68.75 miles per hour. (So, yeah, he'll be breaking the law.)