A candy company needs to project demand for a new type of gum and they think it will mirror an existing gum line. The existing gum line sold $3000$ packets of gum at $\mathrm{.}75.$ The previous year they sold $3500$ units at price of $\mathrm{.}72.$ The candy company wants to know what is the maximum they can charge for the candy and still make sales. They also want to know what is the projected maximum number of units they could sell in a year.