Process costing

I have a doubt, in some process costing sums we deduct the scrap value from cost per unit to get an abnormal gain amount but in some sums, we don’t deduct it, we directly value it, why is it so and how am I supposed to know if I have to deduct it? for eg, if my cost per unit is $10, the abnormal gain is 100 units, and the scarp is $5, am I supposed to do, ($10-$5)x100? or just $10×100?