The till has exactly one trick and it plays it on every purchase: $T = P\,(1 + r)$ (read aloud: T equals P times one-plus-r). $P$ is the price before tax, $r$ is the tax rate as a decimal, and $T$ is the total the register asks for — that last one is what you are solving for. The 1 in the bracket is your price, kept whole; the $r$ is the government's slice, stacked on top. In Ontario the sales tax is 13%, so $r = 0.13$ and every dollar of price becomes $1.13$ at the register — which is why a $100 pair of shoes quietly asks for $113.

The one non-negotiable step: the percent must become a decimal BEFORE it enters the bracket. 13 stays a percent on the sign; $0.13$ is what does the math. Skip the ÷100 and the formula cheerfully charges you thirteen extra copies of the price — the arithmetic never argues, it just obeys.