A. 1600 m
B. 1240 m
C. 360 m
In a topographic map, the contour lines are used to determine elevations, and are made by connecting points of equal elevations. Index contour lines are generally darker and are marked with their elevations. Lighter contour lines don't have elevations, but can be determined counting up or down the nearest index contour line and multiplying by the contour interval, which tells the difference in altitude between contour lines. The contour interval is stated on every topographic map. In this case, the contour interval is 20m. So, to determine the altitude of point Y, we have to count 2 contour intervals from the Index contour line that shows altitude 1200, and multiply it by the contour interval:
2 * 20m = 40m
So, the difference in altitude from the Index contour line 1200 to Y is 40m. We can conclude that we have to add this elevation, because the next Index contour line is 1300, so the elevation at point Y is 1240m.
Point X is part of the Index contour line 1600, so its elevation is 1600m.
To get the difference between the 2 points, we simply subtract X - Y
So, 1600m - 1240m = 360m