To determine the indifference curve leading to the utility level
S = 20, cut horizontally through the 3D utility function
at altitude 20. Project the obtained altitude line onto the wage/employment
surface. This projected line is an indifference curve. Figure 2
redraws this curve in a 2D diagram.
Repeating the same exercise at other altitudes renders other indifference
curves. Those indifference curves situated further north-east represent
higher altitudes (and, therefore, higher utility levels).