A graph. Five curves are graphed on a coordinate plane. The horizontal axis is labeled Quantity per period. The following values are displayed on the horizontal axis from left to right: Q_a, Q_b and Q_c. The vertical axis is labeled Dollars per unit. The following values are displayed on the vertical axis starting from the bottom: p_a, p_c and p_b. The first curve labeled S is upward sloping, begins at the bottom left, goes up and to the right, passes through the points a(Q_a, p_a), b(Q_b, p_b), and exits the top right of the viewing window. The second curve labeled S prime is upward sloping, begins at the bottom right, goes up and to the right, passes through the point c(Q_c, p_c), and exits the top right of the viewing window, to the right of the first curve. The third curve labeled S asterisk is upward sloping, begins at the bottom left above the first curve, goes up and to the right, intersects the first curve at the point a(Q_a, p_a), intersects the second curve at the point c(Q_c, p_c), and exits the top right of the viewing window under the second curve. The fourth curve labeled D is downward