Total utility rises, but marginal utility declines, as consumption
increases. The marginal utility of 20, shown as the second entry in the
third column, arises because Alison’s total utility increases from 30 to 50—
a difference of 20—with consumption of the second juice. To indicate that
the marginal utility is associated with the change from one level of
consumption to another, the values in the third column are recorded
between the rows of the values in the second column.
The data are graphed in the two parts of Figure 6-1.