you ONLY calculate tax on a tax return.
What is withheld from the 401(k) withdrawals are "withholdings", which is simply the down payment against the actual tax calculated on a tax return.
to answer your specific question, the $15,000 of withdrawals are added to you other income on the tax return, the tax is calculated and then the withholdings are deducted from the tax. The result is either you get a refund or owe more.
