Not knowing Canadian law, it appears the original calculation is incorrect. The way you have done it, you have additional taxes because you are calculating 9.5% of the original $500 + the 5% federal tax.

Shouldn't you calculate each tax only on the 500? (500 * 5%) + (500 * 9.5%)

>Here is a common tax scenario that we have seen in many applications. There is a federal tax, which we apply on an amount and then we add on top of this total a province/state tax.
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>Lets take the example of Quebec province. Presently, they are bound to 5% federal tax and 9.5% provincial tax. So, if the amount if 500.00$, we obtain 25.00$ for the federal tax and 49.88$ for the provincial tax. (500.00$*5%*9.5%=574.88$)
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>In a given application, someone has decided to include the total of both taxes in the same field. So, basically, taking the above example, the tax field would then be 74.88$.
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>So, in that application, we do not have a field for the federal tax, we do not have a field for the provincial tax and I do not even have the amount which contributed to give this 74.88$ tax. From the 74.88$ amount, I then need to reverse the process and extrapolate the 25.00$ federal tax and the 49.88$ provincial tax.
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>This should be a simple formula but I just can't find it.