Answer:
Month 4
Step-by-step explanation:
In the first month, Charles City has built 5 km and Tinsel Town has built 21 km
Since it doubles each month, by the end of the second month, Charles City has built 5(2) = 10 km. Tinsel Town increases by 5 each month; by the end of the month it has built 21+5 = 26 km. It is still higher than Charles City.
By the end of the third month, Charles City has built 10(2) = 20 km. Tinsel Town has built 26+5 = 31 km. It is still higher than Charles City.
By the end of the fourth month, Charles City has built 20(2) = 40 km. Tinsel Town has built 31+5 = 36 km. It is now lower than Charles City.