The graph below shows the total cost for lunch, c, when Will and his friends buy a large salad to share and several slices of pizza, p. A graph titled Lunch Cost. The x-axis shows the number of slices purchased, numbered 1 to 8, and the y-axis shows the cost in dollars, numbered 5 to 40. Blue diamonds appear at points (0, 7.5), (1, 10), (2, 12.5), (3, 15), (4, 17.5), (5, 20), (6, 22.5), (7, 25), (8, 27.5).

For each additional slice of pizza that is purchased, by how much does the total cost of lunch increase?