The scale of the map is equal to 250 miles per 1.7 feet. or 250m/1.7ft. set up as a fraction.
Divide 250 by 1.7 to get how many miles are equal to just 1 foot.
250/1.7 = 147.058824 miles per 1 foot
Multiply that times 3.06 feet (the distance between C and D on the map)
147.058824m/1ft x 3.06ft = 450m
The actual distance between City C and City D should be 450 miles.