Joint Products

Two joint products A and B are produced in a process. Data for the process for the last period are as follows: Product A B Tonnes Tonnes Sales 480 320 Production 600 400 Common production costs in the period were $12,000. There was no opening inventory. Both products had a gross profit margin of 40%. Common production costs were apportioned on a physical basis. What was the gross profit for product A in the period? A $2,304 B $2,880 C $3,840 D $4,800 The correct answer is C. To get the correct answer candidates had to negotiate three steps: (i) calculate the amount of cost apportioned to product A ($12,000 x 600/(600 + 400) = $7,200) (ii) then calculate how much of this cost is to be charged against A’s sales in the period ($7,200 x 480/600 = $5,760) (iii) then calculate the gross profit earned using the gross profit margin given ($5,760 x 40/60 = $3,840). Could you explain the step iii of the explanation, please? The 40% should be coming from the $5760, no? Or It's 40% considering both products together?