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@dontwastedollarss wrote:
I thought GST was 10%? how did you get that figure?

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It is 10%, but the equation is price + 10% GST = sale price, not 10% of sale price = GST.

GST is added on to a base price to get a GST inclusive amount. eg if we didn't have GST inclusive prices on eBay and I sold something for $20, then GST was added to the sale afterwards, it would be 10% of $20; which is $2, so final price is $22. You can see that 10% of $22 is $2.20, though, but if you divide the final price by 11, you get 2, the GST component. 

When $139.95 is the GST inclusive price, it's $139.95 divided by 11 = $12.72