The scale can be calculated by dividing the distance in the map by the actual distance. It is to be noted that both distances should have the same unit in order for them to be cancelled out. Converting 28 km to centimeters,
d = (28 km)(1000 m/ 1 km)(100 cm/ 1 m)
d = 2800000 centimeters
Then, the scale becomes 3.5 cm/ 2800000 cm. Simplifying, this will give us an answer of 1:800000.
