Please explain your answer

Divisions J and A are in M Group. Division J manufactures part N. Three units of part N are used in product Z manufactured by Division A. Division J has no external customers for part N. Division J transfers part N to Division A at variable cost ($35 per part) plus 50%. The variable cost to Division A of manufacturing product Z is $50 per unit. This $50 does not include the cost of part N transferred from Division J. Division A can sell the following number of units of product Z, earning the associated levels of marginal revenue. Units sold 1 2. 3 4 Marginal revenue $270 $240 $210 $180 How many units of product Z should management of Division A sell if they wish to maximise divisional profit