Yes, it is. Indeed it is one of two common ways to calculate percentages. Indeed it is the method of calculation YOU insist must be used in certain cases... but it can be used in any case you wish.
CASE: I pay $20 in tax and "the remainder" after paying tax is $80.
There obviously IS a remainder of income after tax, duh.
There is more than one way to figure the rate of tax, as explained:
1) 20/100 or 20% this is the tax inclusive method ie the amount of tax is included in number you're dividing (100 in this case)
2) 20/80 or 25% this is the tax exclusive method ie the amount of tax is not included in the number you're dividing (80 in this case).
Obviously, the dollar amount of tax is the same. The rates are different because each uses a different whole with which to figure percent.
That you insist that this isn't possible after numerous examples is curious.
2) 20/80 or 25% this is the tax exclusive method ie the amount of tax is not included in the number you're dividing (80 in this case).
So where is the other 75%?