it's complicated
examples 1) adjusted gross income consists of $52400 in LTCG then the standard deduction of $12400 reduces taxable income to $40K - your tax is $0.
2)$52410 in LTCG your tax is $2 on the $10 excess
3)interest $42,400, LTCG $10,000 then the $12400 standard deduction reduces taxable income to $40K. the $40K is reduced by the $10K in LTCG so you pay tax on $30K. $0 in tax on the LTCG
4) same as 3 but interest is $52,400, taxable income is $50K, the $50K is reduced by the $10K LTCG so you pay regular income taxes on $40K + 15% on the LTCG
in Turbotax there is a worksheet that computes the tax when you have LTCG and/or qualified dividends
