The following information is about bottles of ketchup: $[$Make sure to show your work$]$

a$.$ Calculate the unit price of an $80-$ounce bottle of ketchup being sold for $$8\mathrm{.}50.$

b$.$ Calculate the unit price of an $18-$ounce bottle of ketchup being sold for $$2\mathrm{.}50.$

c$.$ Which is the better deal $($the ketchup bottle in part a or part b$)?$ Why?