Scale : 2 inches = 39 miles
The length of the highway on the map is 9 inches -> the real length of it = 39 x 9 : 2 = 175.5 (miles)
There are 7 rest stops equally spaced => There are 7-1 = 6 equal distance between the rest stops
So, every rest stop is 175.5 : 6 = 29.25 miles apart from each other.
With a scale of 1 inch = 30 miles, each rest stop will be 29.25 : 30 = 0.975 inches apart from each other on the new map.