Section 5. For the purpose of adding and collecting the tax imposed by this chapter, or an amount equal as nearly as possible or practicable to the average equivalent thereof, to be paid to the commonwealth or to be reimbursed to the vendor by the purchaser, the following formula shall be in force and effect as follows:—

In addition to a tax of five cents on each full dollar, a tax shall be collected on each part of a dollar in excess of a full dollar in accordance with the above formula.