You apple your losses (carry forward and current year) first against any non-discountable gains and if there are any losses still left over apply those against the gross discountable gains. If there are gross discountable gains left over, you apply the 50% discount to them
So in your example:
1. You have no current year non-discountable gain: $0 - $1,500 therefore $1,500 loss to carry forward.
2. You have $5,000 gross discountable gains: $5,000 - $1,500 therefore gross discountable gain for year is $3,500
3. As this gain is discountable, your CG for tax purposes is $1,750.
If you also had company C, which had a gain of $3,000 and held for more than 12 months, this would be the calculation:
1. $3,000 - $1,500 is $1,500 is capital gain, and is non discountable. Losses now fully applied.
2. $5,000 - $0 = $5,000 and is fully discountable.
So your CG for tax is $1,500 + ($5,000*50%) = $4,000
