The graph below shows the amount of money left in the school’s desk fund, f, after d desks have been purchased.
a graph titled desk fund amount. the horizontal axis shows the number of desks purchased (d), numbered 1 to 9, and the vertical axis shows the dollars left in fund (f) numbered 100 to 800. blue diamonds appear at points (0, 700), (1, 590), (2, 480), (3, 370), (4, 260), (5, 150), (6, 40).
for each new desk that is purchased, by how much does the amount of money left in the school’s desk fund decrease?