Math CentralQuandaries & Queries


Question from Monique, a student:

okay i live in ontario and i want to find out what my grand total will be so i know how much i will have to pay...
so if i have a before tax total of $48.40... what will my grand total be? with GST and PST i know that GST=5% and PST=8%

Hi Monique,

Suppose you have a purchase of $A before taxes. The GST is 5% of the before taxes price so

GST = 0.05 × $A

The PST is 8% of the before taxes price so

PST = 0.08 × $A

Thus the total after taxes cost is

$A + 0.05 × $A + 0.08 × $A

This method gives you the two taxes you pay as well as the total cost. If you just want the total cost you can find it in one calculation since

$A + 0.05 × $A + 0.08 × $A = (1 + 0.05 + 0.08) × $A = 1.13 × $A

I hope this helps,

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