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.
