I live in Rapid City and commute to Spearfish. I leave Rapid City by 15 minutes after 8 and get onto I-90 at Exit 52. I must exit I-90 at Exit 12 by no later than 8:50 so that I am not late for class. Normally, I can drive the speed limit and make it on time.

Over the past few years there has been a lot of construction on Interstate 90. For example, last year 4 miles of road was under construction between Mile Marker 26 and 30 (Sturgis). The posted speed limit in the construction zone (single lane -- do not pass) is 55 mph, but I always end up behind someone that is going only 52mph. When I am not in the Construction Zone, what is the minimum speed that I have to drive, so that I can be at Exit 12 by 8:50?