Math CentralQuandaries & Queries


Question from Louise, a parent:

I am doing our business books and could you please tell me how to figure out how to calculate what the 5% GST would be on a total. Such as in example.....$78.79 total figure.....what would be the GST portion. How do you figure it out. ??

Hi Louise.

If you think of the original price as 100% of the original price (sounds rather obvious, I know), and you add 5% tax, then your total is 105% of the original, right? Your total is $78.79, so that's the 105%. You are
interested in the tax portion (the 5%), so you just want the same proportion 5% is of 105%. Therefore, you just multiply it by that fraction:


Punch that into your calculator to find the GST portion of the total.
Stephen La Rocque

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