Marginal Propensity to Save Suppose $C(x)$ measures an economy's personal consumption expenditure and $x$ measures the personal income, both in billions of dollars. Then $S(x)=x-C(x) \quad$ Income-consumption
measures the economy's savings corresponding to an income of $x$ billion dollars. Show that
$$
\frac{d S}{d x}=1-\frac{d C}{d x}
$$
The quantity $d S / d x$ is called the marginal propensity to save.