   SEARCH HOME Math Central Quandaries & Queries  Question from Chris: Dear Math Central, I've been looking at a few answers to the question of before and after GST price calculation. the suggestion have been :: multiply the pre GST price by 1.10 to get the price and reverse to get a pre GST price. But how do you calculate the pre GST price from an existing retail price? For example if you take an item that is $150.00 and multiply it by 0.10 you get$15.00 as the GST add on. Apparently giving you $135.00 as the pre GST price. However if you then multiply$135.00 by 1.10 you get the $148.50 as the total. So the question is how does one calculate the real pre GST price from an existing retail price? Regards, Chris Chris, If the GST rate is 10% and the pre GST price is say$120.00 then the GST is 0.10 × $120.00 =$12.00 and the price including the GST is $120.00 +$12.00 = $132.00. To do this in one step, the price including the GST is 1.10 ×$120 = $132.00. If you know the price including the GST is$132.00 then to find the per GST price you divide by 1.10.

$132.00/1.10 =$120.00

I hope this helps,
Penny     Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.