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why if $700K in LTCG is AGI only $200K. 

$200K less say a standard deduction of about $12K that leaves taxable income of about $188K. and assuming filing status is single. 

About $40K is taxed at zero leaving about $148K taxed at 15%.  so taxes would be about $22K.  but since key info is missing the taxes could be substantially different. 

MFJ about 80K taxed a zero leaving $108K taxed at 15%