I don't understand how to calculate the final column... quarterly for the production budget. Do you just add, or is there another calculation to arrive at that answer?

Here is the question:

VitaPup produces a vitamin-enhanced dog food that is sold in Kansas. The company expects sales to be 13,000 bags in January, 14,900 bags in February, 19,400 bags in March, and 22,000 bags in April. There are 1,300 bags on hand at the start of January. VitaPup desires to maintain monthly ending inventory equal to 15 percent of next month's expected sales. Prepare the production budget for VitaPup for the months of January, February, and March.

VitaPup Production Budget For the months of January, February, and March
	January	February	March	Quarter
selectanitem Unit salesTotal neededBeginning inventory of finished unitsUnits to be producedDesired ending inventory of finished units	enteranumberofunits	enteranumberofunits	enteranumberofunits	enteranumberofunits
selectbetweenadditionanddeduction AddLess: selectanitem Desired ending inventory of finished unitsUnit salesBeginning inventory of finished unitsUnits to be producedTotal needed	enteranumberofunits	enteranumberofunits	enteranumberofunits	enteranumberofunits
selectasummarizinglineforthefirstpart Units to be producedBeginning inventory of finished unitsDesired ending inventory of finished unitsUnit salesTotal needed	enteratotalnumberofunitsforthefirstpart	enteratotalnumberofunitsforthefirstpart	enteratotalnumberofunitsforthefirstpart	enteratotalnumberofunitsforthefirstpart
selectbetweenadditionanddeduction LessAdd: selectanitem Total neededBeginning inventory of finished unitsDesired ending inventory of finished unitsUnit salesUnits to be produced	enteranumberofunits	enteranumberofunits	enteranumberofunits	enteranumberofunits
selectaclosingnameforthisstatement Beginning inventory of finished unitsTotal neededUnit salesDesired ending inventory of finished unitsUnits to be produced	enteratotalnumberofunits	enteratotalnumberofunits	enteratotalnumberofunits	enteratotalnumberofunits