The first part is how many dollars would it cost to build 1000 miles of road. The second part is to determine how much light rail could be made for the same amount.
10 miles of road = $1 million. Multiply by 100 to get the cost for 1000 miles of road.
1000 miles = $1 million * 100 = $100 million.
For the same $100 million, the city could build 20 miles of light rail. (2 miles for $10 million, multiply by 10 = 20 miles for $100 million).