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 Question from Duncan: HI - I've added up the GST from a pile of receipts and would like to know what the total cost of goods would be without GST. I don't want to add up 100's of receipts again with the cost before GST but I do need this total cost. i.e.) I know I paid $\$400$GST, what would the formula be to find out the cost of what I purchased before taxes? Your help is much appreciated! Hi Duncan, Suppose$T$is the total in dollars to which the GST is applied and that the GST is$P\%.$To calculate the amount of GST paid you would take$P\%$of$T.$Thus $\mbox{GST paid } = \frac{P}{100} \times T.$ In your question you know the GSP paid and you want to know$T.$Solving the equation above for$T$I get $T = \frac{\mbox{GST paid}}{\frac{P}{100}}.$ In your example where the GST paid is$\$400$ and assuming the GST is 5% I get

$T = \frac{400}{0.05} = 8,000 \mbox{ dollars.}$

Penny