Charles City has built 5 km5\,\text{km}5km5, start text, k, m, end text of bicycle paths by the end of the first month, and the total length of the paths doubles each month. Tinsel Town has built 21 km21\,\text{km}21km21, start text, k, m, end text of bicycle paths by the end of the first month, and the total length of the paths increases by 5 km5\,\text{km}5km5, start text, k, m, end text per month.