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Some jurisdictions charge tax on top of other taxes, for example Quebec (yes, of course they would!)
For example if GST is 5% and QST is 9%, on a $100 item, then the item + gst is calculated as $105. The QST is then levied on that combined value: 9% x 105 = 114.45, making the tax 14.45% total.
To keep them separate you can subtract the GST as 5% for to get 9.45% for QST when compounded.