*4. Data on before-tax income, taxes paid and consumption spending (on domestic goods and services) for the Simpson family in various years are given below. BEFORE-TAX INCOME ($) TAX PAID ($) CONSUMPTION SPENDING ($) 25 000 3000 20 000 27 000 3500 21 350 28 000 3700 22 070 30 000 4000 23 600 a. Graph the Simpsons's consumption function and find their household's marginal propensity to consume