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 Question from REBECCA, a parent: Hi, I have a total of $\$300.$What is the subtotal & GST? How do I work backwards? Hi Rebecca, Suppose the GST is r%, here in Canada the GST is 5% so I would use$r = 0.05$. Think about going forward. If the subtotal were$\$S$ then to find the total you would add $\$S$and$r \times \$S$ to get the total $\$T$. Thus $\T = (1 + r) \times \S$ To find$\$S$ you need to divide both sides by $(1 + r)$ to get

$\S = \frac{\T}{1 + r}$

so when the total is $\$300$the subtotal is$\frac{\$300}{1 + r}$ and the GST is $r \times \$S.\$

Penny

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