The space between two index contour lines is 500 feet. There are four light-colored contour lines in between. What is the contour interval?
a. 100feet
b.250 feet
c.500 feet

The contour interval is 100 feet.

Explanation:

The index contour lines are contour lines that are thicker than the rest of the contour lines. This is so that they can present better orientation and faster determining of elevation. Unlike the standard contour lines, the index ones are marked with numbers, representing their elevation, so they are used for determining the contour interval as well.

The contour interval basically represents the difference in elevation between two contour lines. From one map to another it can vary a lot, depending on what kind of a map is used, and what's the scale of it. In this case we have the index lines having 500 feet difference in elevation between them. As the index lines are every fifth contour line, that number divided by 5 gives us a result of 100 feet, which is the contour interval.