



 
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  


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