Amap has a scale of 1 cm : 21 km. two cities are 3.1 cm apart on the map. to the nearest tenth of a kilometer, what is the actual distance corresponding to the map distance?

For the answer to the question above,
use the concept of ratio and proportion. The ratio between 1 cm to 21 km should be the same as the ratio of 3.1 cm to the actual distance. Let x be the unknown actual distance and mathematically express the situation,
 
 (1 cm/ 21 km) = (3.1 cm / x km)

This equation gives a value for x which is 65.1 km. Therefore, the answer is letter C.