Both cases are wash sales.
In case A, you sold shares at a loss within 30 days (before or after) of purchasing identical shares. Your brokerage statement should show sales proceeds of $6,000, basis on the 100 shares sold of $8,000 and a wash-sale adjustment of $2,000. As you indicated, your total basis in the remaining 200 shares is increased by $2,000 or $10/share.
In case B, you also sold shares at a loss within 30 days of purchasing identical shares. Your brokerage statement should show sales proceeds of $18,000, basis of $30,000 (the original $24,000 purchase price plus the disallowed loss of $6,000) and a wash-sale adjustment of $6,000. When reported on your tax return, the result will be a $18,000 - $30,000 = a loss of $12,000, plus an adjustment for $6,000 of disallowed loss for a net reportable loss of $6,000.
