State the contour interval used on this topographic map. How are you able to determine the interval?

The contour interval is 20 on this topographic map.

Explanation:

On this topographic map, we can see that the elevation is represented with contour lines, as is in most cases. Not all contour lines have a number next to them that tells us what is the particular elevation, but only every fifth contour line is numerically marked. The reason for this is so that there isn't too much information on the map that will obstruct the purpose of it.

The three contour lines that are numerically marked have values of 100, 200, and 300. As mentioned, the numerically marked contour lines are the fifth ones, so between the one with a value of 100 and 200, there are five contour lines. In order to get the contour interval, we simply need to divide with five the difference between two neighboring numerically marked contour lines.

100 / 5 = 20

The value of the contour interval is 20.