CONSUMER EXPENDITURE The demand for a particular commodity is given by $D(x)=-35x+200$; that is$,x$ units will be sold $($demanded$)$ at a price of $p=D(x)$ dollars per unit.

a$.$ Consumer expenditure $E(x)$ is the total amount of money consumers pay to buy $x$ units. Express consumer expenditure $E$ as a function of $x.$

b$.$Find the average change in consumer expenditure as $x$ changes from $x=4$ to $x=5.$