Answer:
$58,000,000
Step-by-step explanation:
the journal entries should be:
February 4, 4 million shares sold:
Dr Cash 60,000,000
 Cr Common stock 4,000,000
 Cr Additional paid in capital 56,000,000
October 12, 1 million shares repurchased:
Dr Treasury stock 18,000,000
 Cr Cash 18,000,000
Treasury stock is a contra equity account that lowers shareholders' equity, and is reported at the end of the balance sheet. When you repurchase stock, you must record the full value of the transaction under treasury stock since it doesn't result in gains or losses. 
December 30, treasury stocks are sold again:
Dr Cash 20,000,000
 Cr Treasury stock 18,000,000
 Cr Additional paid in capital 2,000,000 
The credit balance of additional paid in capital account = $56,000,000 + $2,000,000 = $58,000,000