Say if I charged you $1000 dollars included GST (5 %). So how much is the amount before GST (5%). What is the formular to calculate this. Thank you
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If the amount before GST is x, then the amount after GST is x+x/20 because 5% is 5/100=1/20.

x+x/20=x(1+1/20)=(21/20)x. If you know the amount after tax then that means (21/20)x, so you find x be multiplying the amount by 20/21, because (20/21)(21/20)x=x.

Example: the amount after GST is $42, then the amount before tax is added is (20/21)42=$40.

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