Math CentralQuandaries & Queries


Question from Donna:

If I have a total amount and would like to know the amount of GST paid (5%) and I would have also paid PST (7%) in the total, can you give me a number to multiply by to get the answer

Hi Donna,

Suppose the amount before either tax was $A. The GST is then 5% of $A and the PST is 7% of $A. Thus if $T is the total amount paid then

$T is $A plus 5% of $A plus 7% of $A

that is

$T = $A + 0.05 × $A + 0.07 × $A = ( 1 + 0.05 + 0.07) × $A = 1.12 × $A


$A = $T/1.12

Once you know $A then the GST is 5% of $A and the PST is 7% of $A.


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