Joint Products- Gross Profit allocation

Hi Sir, I have gone over this question a number of times and I cant seem to figure out how the Gross Profit was allocated instead of being charged full to the product: 2 products A and B are produced in a process. Data for the process for the last period are as follows: Product A:(sales 480 tonnes) (production 600 tonnes) Product B: (sales 320 tonnes) (production 400 tonnes) Common production costs in a period were $12000. There was no opening inventory. Both products had a gross profit margin 40%. Common production costs were apportioned on a physical basis. What was the gross profit for product A in the period? A $2304 B $2880 C $3840 D $4800 The correct answer is C I have been able to calculate cost for product A by ( 600/1000 tonnes X $12000 = $7200) and then I multiplied 7200x 480/600 to get the cost of 5760. To get the gross profit I multiplied 5760*40/100 which would have given me A- 2304. But my answer is wrong as the explanation for the Gross profit is to multiply it by 40/60*5760 which yields C. But I don't understand why. Would you be able to assist me please?