On a map of scale 1:50 000 with contour interval at 20 meters, two places are 4cm apart and one lies five contour intervals higher than the other. The Gradient along a straight line between the two places is approximately?

On a map of scale 1:50 000 with contour interval at 20 meters, two places are 4cm apart and one lies five contour intervals higher than the other.

The Gradient along a straight line between the two places is approximately?

The correct answer is B. 1 : 20

No explanation given