The graph below shows the height an elevator travels, y, in x seconds: A graph is shown with x-axis title as Time in seconds. The title on the y-axis is Height of Elevator Above Ground in feet. The values on the x-axis are from 0 to 5 in increments of 1 for each grid line. The values on the y-axis are from 0 to 75 in increments of 15 for each grid line. A line is shown passing through the ordered pairs 1, 15 and 2, 30 and 3, 45 and 4, 60. The title of the graph is Rate of Ascent. What is the rate of change for the relationship represented in the graph?