I have been paying khums regularly but I now have a question that I am paying about 35-40% government tax where I live. Do I still need to pay Khums? Do I need to pay Zakat? If yes what amount do I need to pay for khums and what amount do I need to pay for Zakat? And I would like to know the reasons for why do I need to pay even I am paying tax. Please give the rationale reasons applicable to this age and circumstances where there are no islamic state etc.

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You need to pay the khoms of what remains from your earnings after all your expenses, including the said taxes.
You do not need to pay zakat, because zakat is paid against certain materials, which you do not have.
Khoms is Islamic payment, that has no relation to the governmental taxes.
Governmental taxes are considered part of your expenses.
Hence, if after all your expenses, you left with nothing, then you do not need to pay khoms.