PMQuestion on ACCA pass paper Dec 16

Jorioz Co makes joint products X and Y. $120,000 joint processing costs are incurred. At the split-off point, 10,000 units of X and 9,000 units of Y are produced, with selling prices of $1.20 for X and $1.50 for Y. The units of X could be processed further to make 8,000 units of product Z. The extra costs incurred in this process would be fixed costs of $1,600 and variable costs of $0.50 per unit of input. The selling price of Z would be $2.25. What would be the outcome if product X is further processed? $600 loss $400 Gain $3900 gain $1600 loss Hi John, ive got this question on the Dec 16 pass paper Question 6, however im not sure how they've come to an answer of 600 loss, as the variable cost per unit of 0.50p a unit they have multiplied by 10000 units, whereas additional cost is only 8000 units Please could you help on this Thanks