Ok ... back to third grade math class.
You owned 200 shares @ 10 share = 2000 cost basis
Now you own 150 shares with a 2000 cost basis(what it was worth at the time of conversion is immaterial to your basis) ... 2000 divided by 150 = 13.34 per share
Sell 15 shares x 13.34 per share = $200 cost basis
