Explanation:
even though it is unusual to use 2 different metric systems in one map, we can solve this easily and without any conversions between cm and miles.
1 cm on the map corresponds to 15 miles in reality.
so, the scaling ratio is 1/15.
now, we use the same ratio for the 4.5 cm between the 2 cities.
x is the real distance :
4.5/x = 1/15
15 × 4.5/x = 1
15 × 4.5 = x
67.5 = x
the distance between both cities is 67.5 miles.